Hi,
I work for a secondary school in the UK, and we are looking for an alternative internet filtering solution. Currently we are using one provided by the local authority. We have very little control over what ports and sites we can have blocked/unblocked. I have done some reading into free or open source internet filters, but would like recommendations for free/open source internet filter software. Ideally we would like it to run on windows server 2008/2012 just to fit into our infrastructure, but we will go linux if we really have to. The amount of NIC's is irrelevant,

Thanks!

You can install and configure a small Linux box as a firewall and blacklist filter. It will be considerably less expensive, both from the hardware and software perspective, than installing a Windows server to do this. There are also dedicated boxes, most of which internally run Linux. Try some Google searches. When you find something that interests you, post what you find here and we can look further for you (capabilities vs. cost vs. company reputation, etc).