P2P over ISP's Router

OK. I know I should know this, but I haven't done ANY ntwrking in about 7 yrs. So, bear with me.

1.) I've got an XP Pro dsktop, and Vista Hm Prem. laptop.
2.) They can both see the internet thru CAT5 connections thru the router provided by my ISP.
3.) I'm not interested in creating a wireless connection on my dsktop, although, my laptop can see that fine.

The problem is I can't seem to create a common P2P where they can both see each other via the router. Is it something to do with Windows F'wall, or what?

PLEASE, HEP ME
GPL

 

they both can't see or is it just Vista can't see if it is just Vista it is permission problem if it is both it most likely is router permissions

 

It's not a router issue. As long as you are on the same network the router doesn't come into play.

Are both computers in the same workgroup?