strange problem......

[chicha]

i formmated my hardisk and installed windows XP just last night.
Every thing is fine but when i try to download some application of the web like gtalk the system simple restarts without any warnings (the procedude is followed i.e it logs off and then it restarts).

No errors are shown after the restart too.
what is the problem?
i did not install any antivirues. please suggest some good ones.

[s18000rpm]

dunno the cause.

but for AV-

free-
Active Virus Shield.

paid-
KAV