Hi, I have a dual boot machine with xp on one drive & vista business on the other. What I would like to do is upgrade vista to win7 pro but still leave the machine as a dual boot without reinstalling xp?

That ought to work, consider that you must have a valid and genuine version of Vista Business if you are using a Win 7 Pro Upgrade (DVD Disc).

Best to make sure that you back up all your important data/files on both hard drive before upgrading, just in case you encounter issues when upgrading.

Here is the information on Microsoft's policy for that type of upgrade:
"In order to install the upgrade version of Windows 7, you must have a qualifying Windows operating system installed and activated. You cannot install an upgrade version of Windows 7 on a blank hard drive. The installation procedure does not ask you to insert a Windows disc in the drive for verification, the actual qualifying operating system must be installed.

If you do not have a qualifying Windows operating system installed with a genuine license activated, then you cannot use the upgrade version of Windows 7 - you would need a "full version" Windows 7 license.

In summary:
1. A qualifying Windows operating system must be installed.
2. The qualifying Windows operating system must have a genuine license (product key) and it must be activated.
3. To upgrade, boot to the qualifying Windows desktop, insert the Windows 7 Upgrade disc in the DVD drive."

Also ensure that the operating systems are mapped onto two separate disk volumes. For example, C:\ and D:\ . If you run a clean install of the Windows 7 upgrade it will wipe the disk volume Windows Vista Business is installed on. If Windows Vista and XP are on separate volumes, this won't be a problem. Always back up, though, as it's better to be safe than sorry in case of the inevitable.