Not sure if this helps, but is it a pairing issue? (i.e. if you go and look at Control panel -> devices and printers does it show up properly)
If not, try this, which I read somewhere and worked for me
a) log in to os x
b) rename the wireless keyboard, in the bluetooth section in system preferences
c) then un pair it with the mac
d) then reboot in windows and try to pair it
Windows seems to work better if you pair them for the "first time"...
Of course, this assumes that you have / can borrow a wired keyboard to operate the machine with while your wireless one is unpaired.