Weird tech question

My work machine has started doing this weird thing where, whenever I hit the Tab key, it responds as if I'd hit Alt-Tab. It's not behaving this way with respect to any other keys--windows don't close when I hit F4, for example. And it seems to have come on gradually--that is, a few days ago it wasn't doing this, but when I tried to hit Tab to post LJ comments from my email, it would jump to the top of the screen instead of tabbing over to the Post button--and now it's doing this instead.

I have no idea if this is a problem with the keyboard or a problem with the OS, and if it's the latter, I have no idea how to even begin to fix it. Does anyone else?

Edit: I hit Alt-Tab a couple of times to see whether or not that would still have the usual effect, and after that the Tab key seemed to work normally again. Thanks to everyone who offered advice, though; I'll keep it in mind if something similar happens again.