messenger makes my pc reboot

My pc has only just started rebooting itself when i am logged on to windows
live messenger, its really wierd because it was fine before an its just
freaking me out. if i appear offline it doesnt restart the pc but i dont want
to be stuck appearing offline all the time, i know that i dont have any
viruses on my pc and thats its too random just to happen now to be a software
problem, the only explanation i can think of is that thes some sort of uber
hacker who only wants to attack my server for some reason.

Please help me