If you have the Upgrade versions of Vista you are supposed to do a full XP install, then install vista over the top.
Most of us dislike these types of dirty installs, instead prefering to do an install to a clean drive or partition.
Here is a workaround:
1. Boot with the Windows Vista Upgrade DVD.
2. Click "Install Now."
3. Do not enter a Product Key When prompted.
4. When prompted, select the Vista product edition that you do have.
6. Install Vista normally.
7. Once the install is complete, restart the DVD-based Setup from
within Windows Vista. Perform an in-place upgrade.
8. Enter your Product Key when prompted.
Original thread @ ocforums: http://www.ocforums.com/showthread.php?t=498314
Just thought I would share. It will probably be patched in a service pack (if they can), the only catch here is that you still have to install over another install. But with this way you don't have to bother with XP. It might be fun to note that you don't even need a different version of windows to install, you upgrade to vista from vista. This was originally posted in the microsoft forums.