The usual culprit is the software firewalls found on most Windoze machines. Temporarily disable both firewalls and see if you get sharing. See if they can ping each other.
If that works, check your file sharing setup. I'll assume both machines are running Windoze XP Home.
formatting link
Another common problem is that a new copy of XP requires that file sharing be enabled. The easiest way is to pick a random folder, right click, select "Sharing" and the wizard will walk you through the process.