Agreed with the above as well. It is only possible to upgrade from 32-bit to 32-bit (ie: moving from Vista 32-bit to Windows 7 32-bit). It is not possible to do an in-place upgrade from 32-bit to 64-bit or vice versa. Here are some resources to help you learn more about 32-bit and 64-bit so you can decide if the move is worth a clean installation - http://windows.microsoft.com/en-us/windows7/32-bit-and-64-bit-Windows-frequently-asked-questions
Hope this helps!
Microsoft Windows Outreach