Did you have anyone remote in and try and work on your machine? These vendors that use remote services often times install a service on your system to connect through. When they connect they get your desktop in 16 color mode, the problem is when they disconnect sometimes your system stays stuck in 16 color mode.
VNC does this, Symantec, Intel.. you have to disable their service, and sometimes uninstall the faux graphic driver they add on. You screen is the stuck in viewer mode, often this is coupled with lost wallpaper and missing themes in Windows.
UltraVNC • View topic - OneClick:Win7/Vista VNC Helpdesk Remote Support System
Some reference on this in the link above.