An unauthorized change was made to windows?

Avast Free Antivirus / Premium Security
1

Hi,

Am running the old Vista (64-bit) OS on a refurb pc for video edit work. The pc arrived with the 32-bit OS so i removed the drive and replaced it with a blank one, then i loaded the 64-bit version. No problems. I downloaded Avast Free to be on the safe side. Then I reloaded the old drive (with the 32-bit OS on board) and reformatted it for extra storage. As soon as the reformat began I started getting the ‘unauthorized change’ pop-up

http://answers.microsoft.com/en-us/windows/forum/windows_vista-hardware/an-unauthorized-change-was-made-to-windows/937b89fc-878b-44c2-b054-dca742a368c9

It says that Avast might (or might not) have something to do with this. Any ideas please?

Thanks,

AV