Block Pornography

Hi there:

I am managing an organizational computer network that uses internet oftenly. Lately I found out that one computer was being used for browsing porn sites that is certainly prohibited by the organization.

How do I BLOCK ACCESS to porn sites and if possible the entire x-rated sites. I have heared that these porn sites are using port that can be blocked. Could somebody advise with this.

For more information. I am using Linksys router as a gateway/NAT of the network. 20 computers, 10 at Windows 2000 Pro, 5 at Windows XP Pro and 4 Windows98se and 1 Windows 2000 Advance Server.

Thanks