OK, a day or two ago, my tab key started to do something extremely annoying. Instead of tabbing from one field to the next on a particular screen, the tab key now jumps me from one open Internet window to the “last viewed” window. I don’t recall doing anything different, and it is unrelated to the various “lock” keys. So why is it doing this? Now, whenever I log in anywhere, I have to mouse my way from the ID field to the password field (and don’t get me started if I’m filling out an order form online). This is driving me crazy! Help, Dopers!
I think your ALT key is stuck. Try jiggling it.
I thank you, my wife thanks you. Hooray!
(happier now 
)
I don’t think it’s physically stuck though. I think Windows just thinks it’s stuck. Windows bug of some sort.
This happens to me all the time. If the TAB key starts acting like ALT+TAB, just flick through your open windows with ALT+TAB and the TAB key on its own will start behaving normally.
I have a similar problem with my shift key. I even bought a new keyboard and upgraded to Windows ME (the only reason I upgraded).
It happens when I am selecting items in Explorer. As you know, clicking on an item, then holding down shift and clicking on another item will cause all intervening items to be selected.
Unfortunately, this often happens when I don’t hold down the shift key. My solution (which I found on the internet somewhere) is to hit the Ctrl, Shift and Alt keys several times. This will clear the ‘stuck’ shift key. It’s annoying and obviously a Windows bug (but then what isn’t).
I hope this will help someone else.