Provide internet security at router??

Hi

I would like to know if there is a solution available that will provide security to users who access the internet via a router.

I am aware that there are solutions which can be installed on a PC, thereby protecting all users of that machine, but I was wondering if such a program exists that can monitor all incoming traffic through a router, regardless of which PC it is destined for.

My network comprises a Win2K server running SBS2K, which is also the domain controller. We have a single active directory domain, and between 25-30 PC’s.

Does anyone have any useful suggestions?

Many thanks