first, make sure your hardware is capable of running 64 bit os.
then you will have to use a 64 bit installer disk.
you will have to do a fresh installation of a 64 bit os.
you can't change 32 bit os to 64 bit.
when you install new os, make sure you have your important data backed up securely.
here's what ms has to say: http://windows.microsoft.com/en-US/windows-vista/32-bit...
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.
To install a 64-bit version of Windows on a computer running a 32-bit version of Windows, you'll need to start, or boot, your computer using a 64-bit Windows installation disc.
If you start your computer using a 64-bit Windows installation disc, but your computer isn't capable of running a 64-bit version of Windows, you'll see a Windows Boot Manager error. You'll need to use a 32-bit Windows installation disc instead.
it says vista but its the same with windows 7 (32 bit vs 64 bit).