Measure available bandwidth on a LAN

Does any one know of any built in tools or safe free tools for XP or Windows 2k3 to measure available bandwidth on a LAN?

I setup a small network with two servers and 7 PC’s running off a Linksys WRVS4400N Gigibit Router and I want to see how the bandwidth is doing over the router.

I would like to see maximum available and current use if possible.

Any help is apreacited!

Cheers,