Lurk R - Have you tried just tapping Alt? I know it seems unlikely, but that really does work for me in this situation.
It can be rationalised as follows...
Your PC doesn't continually 'look' at the keyboard to see which keys are pressed - instead, the keyboard sends unique codes to the PC every time you *either* press a key *or* release a key. In other words, every time you press and release a key, *two* scan codes are sent to the PC. In order for combination key presses like Alt-Tab to work, the PC has to "remember" that you pressed Alt some time ago, but haven't released it yet, and are now pressing Tab.
Now - when your PC has its "hissy fit"
it's entirely possible (this is where you might smell some BS) that the PC has simply ended up "remembering" the wrong state for the Alt key - it thinks the key is still down because it hasn't received the "Alt key released" code. Tapping the key resets everything.
That's my theory and I'm sticking to it!
cbl.
ps Just to emphasise - I'm agreeing with you that this has nothing to do with a physically sticky Alt key - it's an internal Windows problem, which therefore can potentially be solved with a 'soft' approach, rather than a reboot.