I've got problems with Windows 7... It randomly changes to basic colour scheme, and the wireless card is unable to detect any signal from the router. Possible related problems may be that flash keeps crashing on Chrome, which I'm not too bothered about.
Last time it happened I opened task manager, didn't see anything so closed it. However, my PC refused to respond after this - ctrl+alt+del wouldn't work, nor would trying to open the start bar menu. Programs running at the time included Norton AV, ATI CCC, Chrome, GOM player, Boinc manager and steam. After a few seconds a small red patch appeared to the left of my screen.
First and foremost: Legal copy of Windows? If so, it sounds to me like you have serious driver issues. Display and wireless drivers are very common culprits where crashes are concerned. Make sure all your drivers are up to date.