Here are a few options ....
1. Find someone who has a Microsoft installation DVD with 64bit.
Use it to do a clean install. You MUST use your own Vista 64-bit product key
Tutorial for clean install : Clean Install with a Full Version of Vista
2. Google for a Windows Vista 64-bit ISO, burn it into a bootable dvd and use it to restore your Vista. again, you must use your own 64-bit product key.
This is a legitimate source authorized/approved by Microsoft, published by Dell.
2.3 Microsoft Windows Vista Official .iso Download Links (Digital River) Dell Community
3. If you don't mind spending some money, upgrade to Windows 7. In that case, you will be given a new product key for the Windows 7 operating system. Microsoft Store Canada Online Store - Windows 7
P.S. I'm in Canada, so the link is Canadian. You can google for one in your country.