Machine boot properly in windows (winxp). When we login the machine. It automatically reboot without warning. I have replaced RAM. Same problem persists. What should I do?
How about going into bios for a while and keep an eye on the CPU temperature but more so the supply voltages.
The power cord and the power supply unit does funny tricks like this so may be worth checking by substitution...
...I had a hard drive what did this trick, I found it by trying a different drive it didn't have the right mobo drivers but it proved the point, funny part about is it either rebooted, froze or the video blacked out momentarily.... only when I was doing something much needed like a text document, if it wasn't important it'd never do it, the devil just knew.
So in went a new hard drive and that cured it, the fault returned as soon as I put the faulty drive back. Back up any important files just in case this should be the case.
Try running in safe mode to see what happens, whilst in safe mode try a scan for bugs and malware.
Machine restarting continuosly. 
Linear Mode
