Unable To See Network PC In Time Machine

I have been attempting to backup from my Macbook to my PC. I have no problem in general connecting to my PC. I connect to my PC over my wireless network and I can move files back and forth manually. When I attempt to do a backup through Time Machine (while connected to PC wirelessly) it does not see the PC; even though I know that I'm connected to my PC. Even after scanning several times. Is this because Apple wants me to go and buy one of their backup hard drives!?! Which won't happen!