I recently upgraded to Windows 7.0 and I have to say that I'm liking it and it's a heck of a lot better than Vista.  I can't really put my finger on it, it just seems easier to navigate through stuff and get to what I want.  But one of the things I found disappointing was the performance of Windows Virtual PC, which is currently in Release Candidate (as of this posting).  I've got a nice machine with a quad-core and 8 gigs of RAM, and VPCs in the past have been like working directly on another box.  But even with the RAM maxed out for a VPC, the VPC machine was running horribly slow.  I soldiered through working on a VPC for a night, and I think it took about 8 hours to do about 2 hours worth of work.

In the end, I moved the VPC from my host drive (C:) to a USB 2.0 hard drive.  It was like day and night - the VPC was back to running at full speed.  I'm not sure why having the VPC image on your host drive is such an issue (it wasn't in Vista), but if you're having performance issues with VPC on Windows 7.0, I highly recommend moving the image.