You say it was an uneducated thing to upgrade from Vista to Win 7, but not necessarily so. The only problem would be, (if this system was compatible for Win 7 and you had all the drivers for the new OS), is if you tried to upgrade from a 32 bit to a 64 bit OS. That would take a complete wipe of Vista and fresh install of Win 7.

You also said you reinstalled Vista. There's no downgrade path from the higher OS to the lower OS so when you reinstalled Vista back to factory conditions, everything would have been wiped and Vista installed as a fresh install.

That means all programs and applications would need to be reinstalled by you.

It is possible hat some folders remain and these are often renamed as Windows.old for example. May be that is where all the hard drive used space has gone?

Mark