Microsoft has now released Virtual PC 2007, free. It's been in beta since October.
Use virtual machines to run operating systems such as MS-DOS, Windows, and OS/2. You can run multiple operating systems at once on a single physical computer and switch between them as easily as switching applications--instantly, with a mouse click.
What's new is the ability to have 32-bit Vista as either a guest or host, though the emulated hardware is not guaranteed to run Aero Glass. (64-bit Vista can only be a host.) Microsoft also claims "Improved performance compared to Virtual PC 2004".
There isn't a new version for Mac OS X users, and Virtual PC for Mac Version 7 is still for PowerPC-based Macs, and there will never be a version for Intel. Parallels is doing that, for $79.99.
