Late to the party as usual, but hey, that's alright!
So all our virtualization solutions are now available at no cost. That's fantastic news for anyone that uses this stuff day-in, day-out and might have balked at the cost of acquisition previously and just used Virtual Server, because it was free...
I love my Virtual PC. It's the main virtualization tool I use at work, as the copy & paste and drag & drop support make it a winner over the already-free Virtual Server for day-to-day testing. I have a little disk-based provisioning factory that I use to quickly produce domain environments for testing, and it does the job really well.
Get Virtual PC 2004 SP1 (as in, the full product containing the SP1 code, not just the Service Pack) download from the Download Center here.
Unnecessary but-link-and-rumour-and-tip-laden additional information:
Advanced users might also want to play around with Virtual Server 2005 R2 on the same machine to see if it works even better for them... (I do this habitually now - you can just stop or disable the Virtual Server services after installation if they're not in use)
And via Dugie and Alessandro it looks like Windows Vista backups will be mountable in VirtualPC (perhaps not bootable; I don't know, I'm speculatin' here - McLaws suggested that they might even be mountable directly in Vista, which would - what's the phrase? - KICK DONKEY! - if true...)
A smattering of further commentary: