Where do you start....
Win2k and xp are essentially the same as far as networking. If you had mentioned Win2k Server or Win2k3 Server, I'd recommend Win2k3 Server. It offers leaps and bounds compared to Win2k.
If you are worried about the integrity of your network when using the OS's you've mentioned I'd be sure to invest in a good firewall and anti-virus. If your budget is limited go to :
- for a free AV called a2
and look for abtrusion protection or kerio as far as a firwall setup
There are many free resources but its a matter of preference. Just be sure to stay away from BlackIce. Not very reliable. Keep in mind that free software only offers minimal options.
Of course, these suggestions are my personal opinions from my own experiences...