Installing a 64-bit version of Windows Vista on computer that is running a 32-bit version of Windows Vista
If you
have purchased an Upgrade license together with a Windows Vista DVD, you must
use one of the following methods.
Method 1
Purchase a full version of the 64-bit version of Windows
Vista.
Method 2
1. Remove the 32-bit version of Windows Vista.
2. Install Windows XP.
3. Install the 64-bit version of Windows Vista by using an installation method that is listed earlier in this article.
Gee, thanks Microsoft. Instead of just letting me install clean or upgrade my Vista 32 installation, I have to remove Vista 32, and reinstall XP 32, just so Vista 64 can look at it and then erase it anyway.
Assholes.