Basically, I have a working HDD and a fried motherboard. The HDD has a lot of data on it that I really want to keep. The really crucial stuff is backed up, but there's a ton of software and music on it that I don't want to go through the hassle of reacquiring. Also, it's kind of complicated, but ideally I would want to boot from it, to keep stuff like the desktop environment intact and preserve the "feel" of the old computer. So I have three questions:

1. How possible is it to move the old HDD to a brand-new computer? I'm looking to be able to actually boot from it, use the OS on it, and keep the data. Is that realistic?
2. Would it be possible to clean up the registry/drivers/etc. on the old HDD so that it could be booted from on the new machine, but without losing the rest of the data? E.g. could I set it as an external drive on another computer, and clean it up in some way to get rid of any data that would cause conflicts with the new hardware?
3. Barring that, if I replaced just the motherboard and nothing else, would I have to reformat?

Appreciate any advice. I'm sure lots of people have tried to do this.

The only way to do what you want is to buy a new motherboard that is exactly the same as the one that died.

Different brand = big headaches and possible lost/ corrupt data.

If you want a new brand get a new hard drive and slave the old one on the new setup.

You MIGHT be able to do this with what's called a "Repair Install" of Windows. It is a procedure, using your original Win Install disk, to delete old drivers and install all the new ones needed for the hardware on your new mobo. Try looking up all you can on Repair Install on forums.

Be aware, also, that your Windows very likely will refuse to run at first and tell you it failed to authenticate. As it installs, Windows recorts all kinds of details of the harware in the machine. Later it checks that info against what is currently running. If it detects significant changes (and yes, a mobo change with all new components probably will be seen as significant) it "believes" you have tried to install it on a second different machine, violating the license. You will have to phone Microsoft Support and explain it all to them. When they are convinced that you're legitimately just replacing broken components, the can tell you how to re-authenticate your Windows so it's happy to run with no questions.

You don't say if your computer is store bought or home built. There's a big difference in how to proceed. If it's home built you can install the old HDD with a new MB and it will probably boot OK and Windows will reconfigure itself to the new MB. Windows will also give you 30 days to re-authorize the move to a "new computer". Windows also has a utility that will allow you to copy all your preferences to a new HDD if you chose to go that route. As stated, you could install your old HDD as a slave (if it's IDE) or just a non boot drive if it's SATA, and migrate your data at your liesure. Some HDD manufacturers provide a utility that allows copying an old HDD to a new HDD bit for bit. WD is one, but both drives would need to be WD. Now for a store bought box. You will need to buy a new MB from the manufacturer of your box. Maybe it will be the same as your old one or possibly a newer MB. Every store bought box I've ever seen has the BIOS and the HDD boot sectors modified to only recognize each other and not a component bought from a third party. This is how the box makers really make their money, IMHO. This is a major reason why I never buy a computer. Good luck.

uh....no. What will actually happen is that windows will NOT boot, and it will NOT automatically configure itself to the new motherboard. In order to get the new motherboard to work you need either do a repair install (as outlined above) or a fresh install. The problem that you run into in windows when replacing a motherboard is with the Hardware Abstraction Layer (HAL).

The HAL is:
"an abstraction layer, implemented in software, between the physical hardware of a computer and the software that runs on that computer. Its function is to hide differences in hardware from most of the operating system kernel, so that most of the kernel-mode code does not need to be changed to run on systems with different hardware"

So when you change the motherboard, the old HAL is no longer configured to the new mobo which equals problems. What the repair install will do on top of removing all of the old drivers is to remove and reconfigure the HAL for the new mobo. However this isn't bullet proof. I was building a new computer for my brother a while ago and the repair install ended up failing, im not sure why. The end result was that i had to just do a clean install.

If you want to be safe you should probably just move all of your data onto an external hard drive and then do a clean install of windows.

If it is XP, there is a good chance that you will be successful in the migration to a new system, but I would question the further stability of the system with a new motherboard.
Best bet is to start from scratch and import your documents and reinstall your programs on the new system.

The easiest thing is buy a $50 new hard drive and install the OS on that then you can use your old HD for data and not have to loose any data.

That is one way that's hassle-free. However, you can get a new motherboard that isn't the same as the old one and still accomplish what he wants to do, it's just a pain in the arse, and he has to know what he's doing.

There are only 5 different HALs used in Windows XP, and two of them haven't been used on virtually any hardware since about 2001. The 3 currently-used HALs are:

1. ACPI Multiprocessor PC (most common today, with hyperthreaded and dual-core systems being the norm).
2. ACPI Uniprocessor PC (likely if his original processor was single core and not hyperthreaded).
3. Advanced Configuration and Power Interface PC (this is the one that can be a problem, because it's hard to change from this to something else).

If you have an ACPI compatible motherboard (all of them sold nowdays are), then all 3 of these HALs will boot on it, no problem. The only thing that might happen is that if your old installation was HAL #2 or #3, and your new hardware is hyperthreaded or dual/multi-core, your machine will not recognize any cores/threads past the first one. But it will still boot.

A more sinister problem in switching motherboards is the storage controller driver. Your new motherboard must have a disk controller who's drivers are already installed in the old Windows installation or the machine will not boot. It will blue screen with a STOP 0x0000007B error.

The easiest way around that problem is to configure the new motherboard for Legacy IDE controller emulation. Almost all installations of Windows XP have the Standard IDE driver available, and the machine will boot off of that.

Once the machine has booted up for the first time, then it's a matter of removing all the old hardware that doesn't exist (through Add/Remove programs for some hardware, like NVidia video cards; or through Device manager for other hardware -- this sometimes requires some special settings so that you can see the non-present devices).

If you can't get the machine to boot on the new motherboard because of storage controller incompatibility, then you have to do a repair install of Windows. After the repair install, then the machine can boot, and you can proceed with removing all old hardware and installing new hardware. As a bonus, the repair install will change the HAL to the correct one if necessary, so you don't have to do it manually later.

Yes, at some point you'll probably have to re-authorize Windows. A simple phone call to Microsoft will do that, just tell them that the old motherboard died and you replaced it. They will give you a new authorization key without any problem IF you're running a retail version of Windows. If you're running an OEM version, you are hosed and you'll have to buy a new one. OEM Windows licenses are not transferable to new hardware.

If you've run into a HAL problem as discussed above, you can change from HAL #2 to HAL #1 easily using Device Manager. Changing from HAL #3 to HAL #1 is large pain in the arse, but it is possible.

I've moved Windows installations like this before, and as long as you clean up the old devices properly, you will not have any problems. My main home workstation Windows installation is on it's 3rd different motherboard.

I have looked at HAL's before but never that deeply. ^+1 that was a very informative read