I have a problem that’s occuring while I am watching videos fullscreen on Youtube, NBA.com, possibly other sites also, but the problem is that when trying to exit the fullscreen mode it sometimes restarts the PC. On some occassions I can switch between fullscreen and windowed mode without problems infinite times, but when PC doesn’t want to do that it simply goes black screen with that analog or digital signal sign switching (Samsung 940 BF). There’s no error prior or after the restart. It also happened once while in Google Earth, but today it even shutdowned while I was starting GTA 4 – again no error, blue screen, nothing. The games in general work good, without restarts or similar.
Computer is 1 month old: AMD Athlon 64 5600+, 2GB RAM, ATI 4670 512 MB, 400GB HDD, Gigabyte motherboard and 550W of power supply. I have Vista Home Premium 32 bit version installed, with SP1.

I have searched the net for similar problems but couldn’t find a solution.
Thanks for help.