Oh. You're not talking about the virtualization software itself, you're talking about Windows.
Check the EULA. There may be a corner case you can use to justify using your old VPC Windows license, so long as you don't sell or give away the old copy of Virtual PC. I'm not a lawyer, and even if I were, I wouldn't be your lawyer, so you should get your own legal advice on this if you're unsure.
In my case, the company I work for is large enough that it has an inventory of volume license keys. They allocated one for me and I use that. Perfectly legal. Wouldn't have it any other way.
As for Vista, I'm sure I'll be one among the multitude who will wait until service pack 4 or so, and then only if the software I use (mostly TOAD and Office) requires it.
Last edited: Oct 20, 2006