XP refuses to start

I shut down Windows XP normally, after around 3 hours of on-time, and then go to restart around 4 hours later. PC boots up, past the 'DEL to enter setup' screen, and it then comes up with a black screen with the Safe Mode options, Last Known Configuration and Start Windows Normally options. No matter which option I choose, Windows begins to boot up, I get the logo and the scrolling bar, and it then blacks out and restarts as if I had pressed the reset button. I have also found all Setup settings are reverted to original on this restart as well.
Specs of PC available on request.
Please help!

 

no start

Specs would help, also how often this happens & how long have u had the puter. While u gather info 4 us; when was the last time the case was opened & heat sink/fan area dusted out? Mite as well bow the power supply clean also, does this from inside blowing lint back thru the outside. Next unplug power cord then reseat the ram. Power up & post back with those specs..........unless above solved prob.

 

Dusting out didn't work, so I took the initiative and formatted and re-installed Windows, and it appears to have worked. I'm selling the computer in a while anyway, so dusting out and formatting was required anyway. Must have been virus-related. Thanks for the help though!