My windows xp

I got a computer from my brother that had a virus on it, when i got it fixed
and was able to connect to the internet I started recieving messages from
microsoft saying my windows xp is not a genuine copy. I have been through the
windows web site but dont have the money to buy a new copy of wondows due to
the outragous price. Does any body know of a place that I can get a free
genuine copy of windows xp??? I dont think I should have to buy it since it
came on the computer I bought already...