If you have an OEM install of Vista, there's a "built in" hack to upgrade to any version you want, as long as it's the same bits, 32 or 64.
First thing, make a copy of the OEM folder that's found here. c:\Windows\system32\OEM
Second, "find" an OEM key for your brand of computer and version of Vista you want to install. That's the key a company like Dell uses to install, say, Vista Ultimate on ALL their PCs, NOT the key on the COA pasted on the outside of the PC, which is unique.
Third, get a Windows Vista Anytime Upgrade DVD, 32 or 64 bit version to match your original OEM install.
Fourth, download the free program, vLite. Use it to make your own Vista install DVD from the Upgrade DVD, leaving out crap like Windows Defender, stuff for languages you don't know, drivers for hardware you don't have/will never have/can't upgrade (laptops), pre-disable UAC and Simple (stupid) Folder View, hiding filename extentions and other things you'd normally have to fix after installing.
With vLite, you're making a DVD to install ONE version of Vista. Retail and Upgrade Vista DVDs actually contain ALL versions of Vista, even the "N" versions that don't pre-install Media Player and Internet Explorer. The version that gets installed from those discs is controlled by the product key you use.
Fifth. Now that you have your shiny new Vista DVD-R (or DVD+R) and a wiped and formatted hard drive, boot off the disc and install Vista, entering your OEM key when prompted.
After install is all done, you can check the properties on My Computer and it'll tell you you have 15 days to activate Windows. The fix is simple, copy the OEM folder back where it was, then hunt up the script command to reinstall the OEM license file. Go through the Start menu to Command Prompt, right click it then click Run as Administrator, then enter the script command. IT WILL NOT WORK IF YOU DON'T DO THE RUN AS ADMINISTRATOR THING, EVEN IF YOU ARE LOGGED INTO THE ADMINISTRATOR ACCOUNT!
This is a feature that's built into Vista! Each OEM's license file is IDENTICAL for ALL versions of Vista! There's also something in the BIOS of PCs running OEM installs of Vista, so you CANNOT use your OEM folder on a PC built from parts. Ain't Microsoft shooting themselves in their foot a lovely thing? MS can't blacklist those OEM keys without pissing off millions of consumers. Why didn't MS do unique OEM license files for each version of Vista?
The only thing you have to "find" that's not provided with your PC is the OEM key for the desired version of Vista for your brand of PC. OK, you have to know the exact command for the script to reinstall the license file, but the script file is included with Vista- therefore the script file itself is not a tool for circumventing blah blah blah, unless the DOJ wants to arrest MS for providing an illegal hacking tool.
