I've just built up a system for a mate. It's based around a motherboard that i gave him, a Gigabyte GA-8IR533.
He bought some RAM ebuyer 512mb PC2100, CPU Intel P4 2.8 FSB-533(retail boxed), unbranded GF FX5200 128mb, and a new case. We took the rtest of the hardware from his old system.
Now for some reason whilst installing XPpro from fresh on his old drive, it got stuck on 87% during full NTFS format and started making a funny clicking noise. We left it for a few mins to see what would happen, after about 10mins we decided to reboot. This time we did a NTFS-Quick format which worked fine.
We finally installed windows and got to the desktop where after short periods of time the PC would freeze (keyboard and mouse no response). Originally we thought it may be the USB drivers, but it kept happening, sometimes after 2mins sometimes after 20mins... We even tried the old PS2 mouse and keyboard which made no difference. After a while i noticed that the hard-drive light was stuck on... so i assumed it was a the drive. So off we went to the local shop bought an 80gb drive, and all was going fine... until the latter part of XPsetup where it kept locking up.
We changed the RAM to differnet sockets on the board still nothing. He's tested the RAM in a PC at work and it's fine he says.
The only other thing that it could be he thinks is the motherboard or the processor.
Try running chkdsk and do a full surface scan to check if there are any bad clusters on the hardrive or errors in the installation.If you installed windows from a full xp disc then you can also run sfc to check for and replace any corrupted files.
Also make sure all you drivers are fully up-to-date.