Windows validation error is somewhat ambiguous. Is it saying your copy of Vista is not genuine or is it asking you to revalidate which you do by entering your original Vista product key. Have you done that? Many times malware removal will cause core system drivers to be reinstalled. Each time a core driver is installed, a system change counter is incremented. Once a predetermined count threshold is reached, Windows assumes there has been a major system upgrade and you will be required to revalidate with the original OS product key. Sometimes the automatic validator won’t work and you will have to call Microsoft, explain your malware removal activities and they will usually - not always - give you a new product key to enter.

Are you on SP1? There appears to be a problem with Avast 6 and Vista SP1.

Appears the solution is to uninstall Avast which you can do in safe mode. You did post that you can get into safe mode. Then install another AV. I would also run Avast’s cleaner to remove all traces of it prior to installing another AV.

Or install Vista SP2 after Avast has been removed and cleaned, then try to reinstall Avast after SP2 has been installed. I would download the SP2 installer on your other PC and burn it to CD/DVD or copy it to a USB drive rather than doing the SP2 download via WIN updates. Your choice.