Assuming you have windows XP, press F8 on startup to get to a boot menu, select Safe mode... it will display a list of drivers that are being loaded into windows... look at it carefully and if the PC restarts remember which file it stopped at ie AGP440.sys etc etc

If you get into windows, then you should be able to run spyware and anti-virus programs and do a full system scan... you might be able to do a checkdisk/scandisk and defrag etc...

Hi and welcome to Techzonez as well