For me I start with the basics.

Event Log-Virus Scan- Spyware Scan- Quick trip to Windows Update- Check for updated drivers (VIDEO/MOBO)-Bios Settings- Scandisk full- then also if it is freezing then I usually check out these helpful pages.

Then if it was running good at one time- Check for System Restore points-

And for those Really Nasty Hangups...before reformat and reinstall

Put your Xp CD into the tray...cancell out of the installer-

goto the run command type - SFC /Scannow (replace when prompted)

also check the MSconfig for anything starting up that should not be.

Not to meniton also if it is a hangup - try a different theme- Like classic to see if it is a system hangup or not.

Windows Install issues-
Cut back to just the basics-
Video card and cdrom- also try downloading the install floppy disks-
(no sound-nic-or modem-) PNP can take care of those later
Memory- if you got alot of memory- sometimes that can cause install issues-
if you have 2 sticks of 256mb then only use one  
or if you have 2 sticks of 512mb then only use one.