This is a follow-up to my earlier post.
So I tried to update. But the SP1.27 (as I now call it) said the software already was installed. I went into Add/Remove Programs and uninstalled SP1. Planning on rebooting later, I found that the uninstall made all my network adapters unusuable. Normally, the install disables networking temporary, but this time they simply did not work afterwards.
After rebooting, I realized that Virtual PC was completely uninstalled. I installed the RTM version again and applied SP1.27.
Now both Help, About and Add/Remove Programs, Support Information claims that I'm running 5.3.582.27 as expected.