First check to make sure the computers do have different machine names. In XP, the computer name is found in the System applet under the Computer Name tab. In OS X, the computer name is in the Sharing applet in System Preferences.
If the computers really do have unique names, then perhaps you have a user account that is the same name as the computer. This is not a good idea. For example, you wouldn't want a computer named "agedcrone" and a user account called "agedcrone". If you have done this, simply rename the computer to something else.
MS-MVP - Elephant Boy Computers - Don't Panic!