If both computers have Pro Tools LE 5.1 or 5.11, they can be made to be compatable. When starting your session, make sure you checkmark the "Enforce Mac/PC compatability." If you have not done this but have started a project already, you can go to File- "Save session copy as" and it will make a duplicate of the project in another location and will convert it to compatability mode if you specify it to do so.

If going from PC to Mac, the Macintosh will need to have an extension "Joliet" from Digidesign's website under Third party utilities. Download and install this on the Macintosh to allow it to read Windows long filenames.