YEs Windows OneCare does have an AntiVirus. It wasnt bought off of anyone as far as i can remember. There was no announcement on wny aquirement of an AntiVirus company from M$.
So that owuld just lead me to think that they actually wrote the application themselves. Since it is to protect windows who would know better then them. As for the Virus def's it wouldnt be to hard fro them to get their hands on a set and then just switch them to their application.
I will have a look see and find out if they did get this from someone else. I was in the Beta for this long before it went public.