I made this real quick in VB .NET for a friend who wanted to Alt-Tab between Co-Pilot 2003 and Winamp.
It's a small clearish button that Alt-Tabs between two windows.
You can click and drag it around, and to exit - right click it.
You will need Microsoft .NET Framework installed.
You can download it here: Microsoft .NET Framework Version 1.1 Redistributable Package
Switcher.zip
If you have any questions, you can post them on Mp3Car where I posted this program in the forums or email me.
http://www.mp3car.com/vbulletin/showthread.php?t=26272