My compaq 733mhz pentium 3 is crashing almost immediately after starting up, it's a fresh windows xp install so no spyware or anything. How to find out what's wrong?

Was it working OK previously with XP?

Sounds like an old computer. Have you ever opened it up and cleaned it? The Power Supply could be a prime suspect as well if it is not heat.

You can also run the manufacturer's diagnostic on your hard drive.

Will it stay on in Safe Mode?

When you say crash do you mean it totally powers off?

in the very beginning it was ok, like right after installation, previously this computer was running windows 98, the crashing it prints some debug in a blue screen and restarts the computer

Read the post HERE for a few diagnostic ideas and post back any relevant data.