They still have Virtual PC 2007 for those using XP or Vista, however their site's main focus is now on Virtual PC for Windows 7, naturally.

The older Virtual PC 2007 is here.

If you have Windows 7 Professional or Ultimate, you can now boot Windows XP with Windows 7's built-in "XP Mode."  The best part is, the downloads to set up XP Mode includes XP pre-installed so you don't have to find your own disc & key nor worry about installing Windows in the VM. USB support was finally added in this version.

Check out Windows 7's Virtual PC & XP Mode here.