My computer automatically restarts without warning?

On my Windows XP desktop computer I've been having a big problem.

Every time i turn it on, it works for about 5 seconds then automatically restarts. It can barely get to the login screen whatsoever.

And about 50 restarts, sometimes we can log in to the computer for like 2 minutes then it restarts again.

My dad has checked the inside for dust, and there is none so we can rule out that being a problem.

I was wondering what we should do at this point to fix it.

~Note~ System restore does not work, antivirus can only scan the first couple files then it restarts so thats useless too. There is no dust, no overheating.

It has been running perfectly for the last 2 months, and we haven't installed anything new in that time period.

Is there a way to fix it? We really don't want to send it to a computer mechanic because it costs alot so we might as well get a new desktop.

~Emachines~

I thank everyone for the help that they can give.

My computer automatically restarts without warning?symatec

when it restarts again, then try to go to safe mode. you can get that by pressing f8 before the windows loading screen comes on. then try to run a scan. or try to do system restore in safe mode. it might work.

also if it doesn't ask your computer manufacturor. the help is usually free like dell in the phone...