for the heck of it..run adaware...maybe it is some evil spyware (I doubt it but may as well eliminate that possibility)
www.lavasoftusa.com get the free version..update it, run it.
Otherwise it sounds like something one of those custimization/registry hack utilities would do.
Probably just a registry thing which can be undone.
You have no clue how it happened?? Did you install anything before you noticed it?
Could it possibly be related to some video card software? Got a new vid card lately??
JP