With a support note quietly posted to its Web site, Microsoft confirmed what enthusiasts have long feared: Users who purchase Upgrade versions of Windows Vista will not be able to perform clean installs of the operating system. Instead, they will need to first install their previous OS and then upgrade in-place to Vista.

"This problem occurs because Windows Vista \[Setup\] does not check upgrade compliance," a support note on Microsoft's Web site reads. "Therefore, you cannot use an upgrade key to perform a clean installation of Windows Vista."

While this is sure to infuriate some users--and certainly, it contradicts information Microsoft provided me with last year--the reality is that upgrade installs of Windows Vista essentially wipe out the OS and perform an install that is very much like a clean install. The real problem here, therefore, will be the length of time it takes to install Vista using the Upgrade media: Though Vista often installs in about 30 minutes, previous versions often take twice as long.

UPDATE: I haven't tested this yet, as I don't have Vista Upgrade media to test, but I'm told that Microsoft's internal documentation does explain how to clean install Vista using an Upgrade version. It appears to be more of a workaround than a true clean install, however. Here's what it says.

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.