1. The CPU heatsink has fallen off. Fix that!
2. The motherboard or something else has BAD CAPS. To learn what these are and how to inspect for such type BAD CAPS at google.com and make the inspection.
In closing you didn't give make, model, age, story or much to work with so my ideas are what's sending machines like that to the service counters.
This happens only once every 3 weeks or so. Rather than booting to XP, I get a black screen with a small white underline at the upper left - as in MS-DOS. I resolve this by shutting off power, waiting a minute, and powering up again while holding down F8. I then usually get a menu from which I can choose to start Win XP. One time the computer showed a blue screen and made a loud alarm-like noise through the speakers, with text indicating a possible hardware problem. Shutting down and starting again, it booted normally.
I've been told there may be a problem with the BIOS, but I'm not familiar with this and have never accessed it.