Just as a heads up, I'd recommend using this code instead if the reason you want to disable alt+tab is for gaming. I needed to disable alt+tab so that I could use push-to-talk (alt) and look at my score (tab) without switching out of the game. Simply disabling alt+tab will prevent me from checking my score while I'm talking. However, this autohotkey code will send tab-down when you press alt-tab (without releasing alt) and tab-up when you let go of tab - which solves my problem.
Code:
LAlt & Tab::Send {Tab Down}
LAlt & Tab Up::Send {Tab Up} P.S.: Sorry for resurrecting an old thread. I posted this here so others that searched "disable alt tab" in Google could find this, as it was one of the top results.
EDIT: I just noticed it's not all that old... Also, here's a link to the AutoHotKey page instead of a direct link to the download (download button on right side of page last I checked):
http://www.autohotkey.com/