I'm sure you've already tried this but it doesn't hurt to clarify.
1.) Ensure you have Windows set to connect to the router the way the router is expecting to be connected to. (Example: If you have a G router, make sure your wireless adapter is set that way)
2.) Ensure Windows has the correct WEP key (if you have WEP enabled on the router) and or a password to connect to your router.
3.) Ensure your router isn't set to allow specific MAC ID's access, if it is set this way, add the MAC address of each of the Windows wireless cards. (This may not be an issue as the Macbook can connect through OSX. Technically, the MAC address is set at the hardware level so it wouldn't change regardless of OS.).
4.) What kind of router is it? Should be able to research any issues regarding Windows 7 and that router.
Just a few steps to think about while you wait for other responses