I had XP compatibilty mode running on my Windows 7 Professional 64-bit machine awhile back. I then uninstalled it to save some space. Today I downloaded it again from here (followed the directions):
and I installed Windows XP Mode, when I try to install Virtual PC or XP Mode Update I get a dialog box that states that these updates have already been installed. I restarted my machine but do not have Windows Virtual PC as an option in "All Programs".