You can't upgrade from 32bit Windows XP to 64bit Windows 7, but you can do a clean install
QUESTION I have recently upgraded my PC from XP to Windows 7 Ultimate 32bit edition. I understand that the disc and activation code can be used again, so how can I activate the 64bit version using a 32bit-labelled disc? Ron Hassett
HELPROOM ANSWER There is no way to perform an upgrade from a 32- to a 64bit version of Windows; instead, you need to complete a clean installation of Windows 7 64bit. However, if your disc contains only the 32bit edition, you can't use it to install 64bit Windows. (See also: Run 64bit Windows XP on a Windows 7 PC.)
If your copy of Windows didn't come with two discs (one 32bit, the other 64bit), you may find your product key isn't valid for a 64bit installation. This will depend on the exact version of Windows 7 you purchased and whether it's an upgrade, OEM or a full retail copy. Full retail copies should include a code for 64bit installations, but you'll still need the relevant disc – perhaps a friend has one you could borrow.
If you haven't already checked, ensure your processor supports 64bit operation.
If you're not sure, use the free CPU-Z utility. Look for support for EM64T in the section marked Instructions.