Potential malware restarts computer, blocks internet

Greetings,

I have the following problem: few days ago computer started to frequently restart by itself, basically every time, I run some larger program. Also it is not possible to open any web pages (neither in Mozilla nor Chrome) - any page opened shows that it has crashed. Both browsers crash after ~minute of trying to use them.

Interestingly enough, I was able to watch movie for 2 hours, while running any larger program (game, running AV scan), immediately restarts the computer. I brought (by flash) Malwarebytes, installed, and run, but everytime when starting the QuickScan, computer restarts.

When trying to run computer in safe mode, it freezes when loading drivers at mup.sys (Internet search revealed to me that this could have many causes).

Any suggestions?
Is there a way how to force it run safe mode, or to run some anti-malware tools without restarting (i've unchecked the automatic restart when something crashes, but no change)?

Would reinstalling WinXP help - or would it worsen the situation if SP3 and antivirus won't work.

I'm running WinXP Pro, with WinXP Firewall and Avast AV.

P.S. Sorry, if it sounds stupid, but i'm completely new at this problem. I've been living clean for years.