Restarts when booting??

Hello,

I've just bought a used computer with Windows xp professional sp2. It worked just fine yesterday. I installed programs like Opera browser, Battlefield vietnam and Steam. But now today, when i tried to turn it on something strange happened.
When i booted it it would look just fine, until it had loaded and all, then suddenly it just restarted and did the process all over again. I've tried using the F8 during booting, to make it go safe mode and all that stuff, but it just wont work!

I dont have the xp cd :(

Can anyone tell me what to do? Please... I'm very frustrated with this.