To me, it sounds like you are logged into both XP computers with different user name accounts. For example, maybe you are logged into the 1st XP box as BOB, and as TOM on the 2nd XP box.
When you try and go to \\XP2 from \\XP1, your user account (BOB) is sent to \\XP2 for authentication, but \\XP2 does not have a user name of BOB, and the authentication will fail.
This is easy to fix. And there are a few options. The easy way is to make a user account of both machines with the same user name AND password (call it JIM with a password of "password"). Log into BOTH machines with that username. Give this JIM account appropriate rights on both machines.
THEN try and go to \\XP2 from \\XP1 while logged in as this new account. This way, the user account of JIM will be passed from one machine to another, and it will match up on the target machine, with the correct password and permissions, and you will be in!
Another way (the way I actually would do it, but I am a Windows geek like this!) is to log into each machine with whatever account you like, and MANUALLY authenticate to the target machine BEFORE trying to connect to access a shared folder or shared printer or whatever.
You can manually authenticate using the NET USE command from the Command Prompt, hitting the IPC$ on the target system, and specifying the user credentials to do this....
If NONE of that made any sense, then go with the first option! If it did sound slightly familiar, let me know and I can explain in more detail!
hope this helps