weird error when connecting

okay i got a big problem... everytime i start steam and try to connect a server it crashes. after that a msg pops up asking me to reboot my pc. now when i try to connect again it works, this really annoys me. does anyone got a clue what causes the error?

You got a virus, and everytime you push the "reboot now" button in the error message, your executeing the file... and spreading the virus.
Scan your steam folder, and if that dont fix it un-reinstall steam...

Or try to google: "Steam crash virus" or something...