I just got my PC formated..n i am facing a Problem which my Wender is also not able to understand

My PC's config. are
OS Windows XP
P4 1.7 GHz
1GB RAM
256MB Grafic Card (NVedia GForce 5500X)
The problem is...My PC restarts when ever I press Alt+Tab...and somtimes it restarts while the game is running..
I have Repaired XP also..but still i am facing the problem...can any 1 suggest me what should i do??
REPLY SOON