I have BF2 but have not played online in quite some time. I performed the following to get my BF2 GUID:
# Enter a BF2 Server that is using Punk Buster.
# Hit the (`~) key to bring up the console.
# Type in pb_myguid
# Copy down the result. This will be letters AND numbers.
Hope that helps.
* Edit *
Additionally, you can place the attached file (pbcl.cfg) in your pb folder which will enable Punkbuster client logging. When you join a server, it will capture your GUID (when you enter the pb_myguid command). If using Windows XP the game install path is probably C:\Program Files\EA GAMES\Battlefield 2\pb.
pbcl.cfg