Microsoft has never supported the downgrade process, and an upgrade is out of the realm of System Restore. Thus, unless you happened to clone/ghost your hard drive using third-party software there is no way to revert to Home Basic. Your only option is to perform a clean installation, wiping out all of your files and settings, but for that you need the recovery system discs. Thus, you must do the following:
1.) Contact the PC's manufacturer and request they send you a set of recovery discs, usually at a nominal (~$30) fee.
2.) If you have files on the computer you need/want to keep, let us know and we'll help you recover them.
3.) Perform a clean installation using the recovery discs.
4.) Keep a baseball bat handy for the next time someone touches your computer. 
John
How can I remove a Vista Home Premium upgrade on a computer that came with Vista Home Basic preinstalled?
Someone installed a Vista Home Premium upgrade that was never activated. It appears that the grace period for activation has expired because this computer boots directly to the activation menu and you cannot access Windows except to activate the upgrade.
I do not want to activate the upgrade. I just want to remove the Vista Home Premium upgrade and restore the computer to Vista Home Basic.
I tried booting from the upgrade CD but there doesn't appear to be any removal option available.
I would prefer to just start again with a clean Vista Home Basic install. Unfortunately I do not have any recovery CDs.
Thank you.
Chowhound
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