http://windows.microsoft.com/en-US/windows-vista/32-bit-and-64-bit-Windows-frequently-asked-questions
Can I upgrade from a 32-bit version of Windows to a 64-bit version of Windows?
No. If you are currently running a 32-bit version of Windows, you can only perform an upgrade to another 32-bit version of Windows. Similarly, if you are running a 64-bit version of Windows, you can only perform an upgrade to another 64-bit version of Windows.
If you want to move from a 32-bit version of Windows to a 64-bit version of Windows Vista or vice versa, you'll need to back up your files and then choose the Custom option during Windows Vista installation. Then, you'll need to restore your files and reinstall your programs. For more information, see Installing and reinstalling Windows Vista.
For detailed information about installing and upgrading 32-bit and 64-bit versions of Windows, go to Installation choices for Windows Vista (32-bit) or Installation choices for Windows Vista (64-bit) online.