Network computers through router

Help please,

I have two computers, my desktop is connected via ethernet to a Netgear DG834gt router. My lap top also connects to the same router using the wireless connection. I have a printer ethernet connected to the router that works fine for both desk top and lap top.

Now my desktop is running Ubuntu Feisty Fawn and connecting to the Internet without problem (ethernet). My lap top is running XP pro and also connecting to the internet without any problems (wirelessly).

I want to be able to network the computers together so that I can share folders (I used to do this when I was running windows on both computers).

The problem is I do cannot see my laptop (XP Pro) from my desktop (Ubuntu).

Can someone please explain how to network the computers in clear steps.

[Note under System - Administration I can see Network but there is no Network Manager - is this right?]

Thanks
Denise