Paul Thurrott posted a guide about upgrading 32-bit Windows Vista/XP to 64-bit Windows 7

While there were once valid reasons for skipping 64-bit versions of Windows, those reasons have disappeared over the past year or so. As a result, it should come as no surprise that many Windows users are now interested in upgrading their 32-bit version of Windows XP or Vista to a 64-bit version of Windows 7. And because all retail versions of Windows 7 Upgrade come with both 32-bit and 64-bit Setup discs, it would seem that doing so is both supported and straightforward.

Well, it's neither: Microsoft does not support a traditional, in-place upgrade of any 32-bit version of Windows to any 64-bit version of Windows. (Well, I guess that statement is technically straightforward. So maybe they're 1 for 2.)

Upgrade from 32-bit Windows Vista/XP to 64-bit Windows 7