If you’re using Virtual PC 2007 on Windows Vista — you may see an error from Virtual PC that states that it cannot launch your virtual machine because the files are marked read-only (but you know the files are not marked read-only).
What you’re seeing is a side effect of the new UAC support in Windows Vista. Now — before you go racing to disable UAC (which would be a bad idea) — the fix for this is simple. Find the virtual machine files, open their properties and view their security settings. Most likely system administrators will have full control of these files.
But UAC stops Virtual PC from using your system admin credentials to open the files. So you will need to edit the security settings in these files and specifically grant full control to your user account. Once you have done this everything should work correctly.