I recently built a computer for a friend:

specs are:

intel q6600 quad core
4gb AData RAM
8800 XFX GT
600W ultra x-finity psu
Gigabyte S-series P35-DS3L Mobo
750GB WD HDD
XP Pro 32-bit

The problem he's having is that sometimes the computer will hang on the POST screen. It'll display the gigabyte logo, but won't proceed any further. Generally, if he powers the computer off for a time, say 15-25 minutes, then it will boot up and work fine. I'm not exactly sure what the problem is as it is not always consistent which makes it slightly harder to diagnose. What should I do?

Check the RAM, start with one stick, the PSU, and run the hdd mfg's diagnostic/repair software. If none of this, reassemble it out of the box and make sure you have the precise number of standoffs and each is screwed down Test after installing each part

Is it overclocked? Was bios updated? Clear cmos & start over.

I had the same intermittent problem on a system i have that i used a spare, marginal (low amps 12V rail) power supply. I changed out the power supply to one with plenty of juice and the problem went away. U might want to check that the cpu and video card are not on the same 12V rail on your system. which rails the connectors are on are indicated in the manual for the 2 ultra psu that i have.