It's the same agent as for a Server, but you don't need to license it, just run a silent install on the XP machine
See
http://support.veritas.com/docs/245171
http://support.veritas.com/docs/270139
There are a bunch more TechNotes concerning XP. Go to the search page and enter "XP", filter for v9.1 and TechNotes" to see them all