Network bandwidth utility...recommendation?

I need a program that can tell me exactly how much bandwidth i have between two building connected over a wireless bridge.

I need to know the maximum bandwidth available and track what percentage is used over a few days.

We are running an all windows network.

Anyone know a utility that does exactly this? Preferably a free one, or one that will work for a few days before you have to register. Please help!

Thanks!

Matt