Hyper-V Program Manager
This is very cool. To help web developers who have IE 6.0 or 7.0 installed on their computers, and need to test the alternate version, the IE team have provided virtual machines with Windows XP and IE 6.0, and Windows XP and IE 7.0. You can find out more details (including download links) here.
Virtual PC 2004/2007 offer the excellent feature of differencing virtual disks.
It's very easy to create a setup based on one single installation:
Create a virtual disk with ie6, create a differencing virtual disk that is based on the vhdd that holds ie6 and install ie7.
The advantage is less disk spaced needed.