is there a windows COA sticker on the computer anywhere?
I assume you have been told by the windows genuine advantage thingy that it is not a genuine copy?
If so then you may be ok, I've seen some really badly setup new PC's that fail the windows genuine advantage validation but when I've checked they are not using the same key as the one that is on the sticker, change the key and problem solved...
check the key your current windows has been installed with and compare with the one on the sticker.
http://www.magicaljellybean.com/downloads/kf14 1.zip
If they are different it's possible to change the key.
If you have no sticker, you've been done, you need to buy another copy.