How can i speed up my dell opitiplex 755

Well, speed it up in what? Overall speed, general tasks, rendering, gaming, photoshopping?

You have plenty of RAM for what most people use. However, if you have a 32-Bit OS installed, then you won't have access to all 4 GBs. Upgrade to a 64-Bit version of windows (if thats the OS you use). That way all 4 gigs can be utilized.

Also, you have a tiny HDD. I'm assuming its probably getting pretty full, which will slow down system performance more than you'd think. Upgrade to a 500 GB 7200 RPM HDD with lots of cache. I recommend this one: http://www.newegg.com/Product/Prod [...] 6822136795

Your processor is fine, but you can't overclock it due to it being a Dell. I have a 3.0 Ghz core 2 duo, and I multitask like a mother, and it works fine for me.

The problem with gaming side of performance increases, is that you have a crappy PSU, which will severly limit what GPU you can get. I'll let someone else find a compatible GPU that will work with that PSU.

Also, make sure you defrag the HDD once in a while. If you have never done it, it can slow your computer down as well. Make sure you have a good anti-virus program that you can shut down. This reason is because an active anti-virus is really good, but can slow your computer down a great deal. If you shut it down during normal use, and start it up when you want to scan, that works.

You can also go into msconfig and tell both cores to be used during computer start-up, as well as disabling what programs are automatically started up by your computer. A decent guide is here: http://netsquirrel.com/msconfig/msconfig_vista.html . The process is very close to the same in Windows 7 and in XP.

There are a number of reasons that may causing a computer running slow, but generally two factors: hardware part & software part.

For hardware part: upgrade the memory, hard drive (for larger volume as 80GB is really rare now)... will cost a lot but proven to be most effective.

For software-part: clean junk files, caches, registry errors... about 80% slow PC caused by this reason. In this case you can use this Dell speed up tool to deal with the issue.

Hope this helps, regards.