networking the internet

we have a network set up.
The thing is that we can both see each others computers but the guest (im the host - with the modem) cannot access the internet. we have set up internet connection sharing etc but we still cant get it to work.
we did have it working but i had to reformat my computer and ever since then we cant network the internet.
we are both using windows 2000 professional.
thanks heaps !!