how do I connect 2 pc's (to each other) ?

[#1]

I have googled and searched and asked support but I'm getting nowhere and am baffled by all the confilicting stuff I read.   Perhaps someone out there knows the answer....
I've got 2 pc's (one on W2000pro, one on XPhome).  They each have a network card AND a wifi adapter, so both can connect to the Plusnet router by RJ45cable, or wirelessly), so I get the internet on both, simultaneously.   Both pcs are set to "obtain IP address automatically" if that means anything.
All I now want to do is be able to view/copy files between the 2 pcs. ("map"  a network drive ?)  
Can I do it without affecting the internet connections ?
Do I need anything in addition to the router ? (a switch ? a hub ?)
Doesn't a router work like a hub?
Can the 2 pcs link by wifi ?  (using the router ?)   (not using the router ?)
Is it just a question of settings ?
Should I just forget it and go down the pub ?

[#2]

It is just a question of settings and having both computers set to use the same workgroup
http://www.homenethelp.com/web/howto/HomeNet-Win2kXP.asp

[#3]

Same workgroup and having exactly the same user names and passwords on both machines makes it easier.