I have some pretty old virtual machines that I use extensively for development and testing.  Recently I upgraded to Virtual PC 2007 SP1 and wanted to upgrade their Virtual Machine Additions, but when I attempted to uninstall the older software I was greeted with the following dialog box:

Well, I immediately clicked OK, knowing full well that a source of "1" was invalid just to see what would happen.  Sure enough, it would not uninstall.  Browsing for the source prompted for the "Virtual Machine Additions.msi" file which I don't have - at least not the version it's looking for.

I had no recollection as to which version of Virtual PC was used to create this particular .vhd file, but I knew it wasn't recent.  To fix it, I had to locate the correct version of the VMAdditions.iso, mount it, and then uninstall.  This is how you might go about fixing it:

1. Locate Virtual PC 2004 SP1 and download it (I didn't have it anymore).
2. It's a .zip file, so extract it to a folder (i.e. c:\vpc2004).
3. Don't install it, we don't want to mess up our existing installation of 2007+.  Instead, perform an administrative install of the .msi file which will prompt you for a network location (a local path is fine) to extract the contents (I chose c:\vpc2004\extract).
   msiexec /a "Microsoft Virtual PC 2004 MSDN.msi"
4. Then you can navigate to the appropriate location and retrieve the VMAdditions.iso file.
   C:\vpc2004\extract\Program Files\Microsoft Virtual PC\Virtual Machine Additions
5. Then, within your VPC, capture the VMAdditions.iso file, cancelling any installation process that may start via Auto-Play.
6. Uninstall the previous software.

Not too painful, but definitely an annoyance.  Now, I've since archived off the .iso and .vfd files from the 2004 SP1, 2007, and 2007 SP1 versions of Virtual PC just in case I have to do this again in the future.