Well, you're lucky, updating the graphics driver didn't work for me. Which graphics card do you have?
I've updated almost every driver on my system (graphics, sound, network, touchpad), I've removed every program that was pre-installed by Dell (I have a Dell Studio XPS 1647), and I even did a BIOS update, but the result is always the same: Alt-Tab not on top.
I also have no IntelliPoint or ThinkVantage Communications Utility installed, which was described by some others here to be the reason for this strange Alt-Tab behavior. The only solution which works reliably on my computer is either closing all sidebar gadgets
or uninstalling the IE9 update. With IE9 removed it works fine, even with sidebar gadgets.
So, I really tried everything that was suggested by others to help, but Alt-Tab is still broken. It would really be nice to know if Microsoft is aware of this very annoying problem and trying to fix it. I mean, even if it is a driver problem and not IE9, I
could not find the driver responsible for it, and I won't be able to find it without support from here.