LOL sex change got me to look at this thread
Only thing I can think of off the top of my head is something could have become changed in Cmos... the settings that are used at bootup. Usually you get a message to press an 'F' key to enter cmos at powerup (before windows or anything else starts to load).
Some people may not see or notice it, it usually only lasts a second and it's the very first thing you see (usually). If your moniter is slow to react on powerup you may never see it. (can't say i've ever experienced that but...)
Sounds like a wierd problem, but if you have done nothing that you can remember to cause this, it could be something like this.