Problems setting up internet sharing between MB and windows desktop

Hi all.

I've just aquired a windows desktop pc and due to placement of the router, want to run the internet on it from my macbook.

I've set up the sharing of the internet on the macbook in the system preferences fine, as windows can detect the local area network, but for some reason can not access the internet on the machine.

Any ideas as to why, would be fantastic.

Thanks in advance for any help
Stefa