How do I isolate a computer from the internet when...

Question

I have a Sitecom wireless router that claims to combine an ADSL 2+ modem, a router, an Ethernet switch and a wireless access point.I want to use this router to connect my computers to broadband and also allow them to share files with each other - this is the easy bit!I also want to connect my 9 year old computer to the network mainly to act as a print server. This may be difficult because my old computer (excellent though it is) can no longer support modern firewall / virus protection software.Can anyone tell me how to configure the Sitecom router so that my old computer can act as a print server and also be isolated from any malicious stuff coming from the internet?