Vista Ultimate downgrade to Basic or Premium.

Hello,
So, I bought a computer from a "friend", he put a whole bunch of random, but good parts in my computer to fit my needs. There was Windows Vista Ultimate already on the computer... but, almost a month later, when I turn on my computer a message pops up saying that it is not a legitimate copy of Ultimate. I can't get in touch with this "friend" any longer, and was wondering if I could "downgrade" from Ultimate to Basic or Premium, without having to reformat my computer. (I do not have any of the software that came with all of the componants in the computer).

(I can't afford to spend money on the Ultimate)