I use Virtual PC a lot, and it stores the full path, including drive letter, to saved state and hard disks.  This is a real pain in the arse when you plug in multiple thumb drives and USB devices, and Vista changes the drive letter... unless you can get back to that drive letter, you lose your saved state.

Thanks to this post, I was able to change the drive letter for external drives in Vista and get my saved state back.

To change the drive letter or paths in Windows Vista, just follow these simple steps:

1. Right click on Computer and select Manage to open and access Computer Management Console.
2. Navigate to and expand the Storage section, and click on Disk Management to access Disk Management Console.
3. Disk Management Console display the details of all available volumes and disk drives in the computer configuration in the right pane.
4. Right click on the volume or disk drive that you want to change the drive letter, and select "Change Drive Letter and Paths).
5. Then click on "Change" button.
6. Use the drop down list to select the new and your desired drive letter to assign to the disk volume.
7. Click OK.
8. Click Yes to confirm.