Hunting Lite

If you've got some time to waste, go to the Windows Marketplace and download Hunting Lite.
It's a demo of a Tower Defense with a lot of charm. Unfortunately it's only in VGA resolution, so you'll see some black bars left and right, but it's not that big of a deal.You can only select 2 Levels on easy, but you can't complain if it's free, right?

The full version only costs 1,39€, so that's pretty cheap if you want more Tower Defense fun.