continuous reboot

comp running win xp pro 1.6 ghz pentium 128 mb ram wd 40gig hd. get bsod and immediate reboot, ( too fast to read) even in safe mode. no virus as I have mounted another drive as master and slaved orignal and run a scan on entire drive. all files accessable from new drive but would like to find out problem and rectify on original with out reformat. this is friends comp so don’t know exactly how problem started. she tried system restore but it wouldn’t work. reboot occurs just as desktop finishes loading. have tried repair from install disk but don’t really understand how to use it. did mbr repair to no avail. also put different memory in.