בלי לפגוע בקבצים הקיימים? או צריך להתקין מחדש הכל?

http://windowshelp.microsoft.com/windows/en-us/Help/41531554-d5ef-4f2c-8fb9-149bdc5c8a701033.mspx

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 Vista, you can only perform an upgrade to another 64-bit version of 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.

If you want to move from a 32-bit version of Windows to a 64-bit version of Windows Vista, back up your files and perform a clean installation of the 64-bit version of Windows Vista.