Isolating one computer yet sharing printer and data transfer

In my business, I am getting fed up with Windows installing patches, Norton software slowing down everything, etc.

Thus I've decided to build a second PC and only use that for emails and internet and leave my main PC for work processing only. I'll remove Norton from this PC and install it on my email machine.

I've bought a KVM switch so I don't have to duplicate some hardware. The only problem I see is with sharing my HP 2600 LAN printer (and also data transfer between the computers). I'd assume that if I network the two together, then my main work PC will still always try to access the internet. So can this be disabled (and thus keep thus PC free from spyware, hackers, etc). Should I install a firewall card or what?

Currently the main work PC has XP on it, but when I go to this setup, I'll probably migrate to Vista (as I plan to do a clean HDD install and only install essential work related software).

Any advice is greatly appreciated.