with the power off...and unplugged open your case and chech that all the ram and cards are seated. if you have more then one dimm try removing one and see if one went bad. if system still blue screens then swap the dimm if it still blue screens try booting from a hirem boot cd min xp and see if it fine..if it boots from a cd and does not blue screen run a anti virus sweep and then try booting into safe mode. if you cant you may have to nuke and pave windows.