Make sure that you can connect to your desktop computer; seeing it listed and connecting to it are different things. Make sure that none of the firewalls, or security software are blocking file and print sharing. Once that is done, start the add printer wizard, and check the 'network or attached to other computer' button, but don't try and browse for the printer, rather use the 'connect to this printer' address box, and enter the info manually; it is usually at:
\\Desktop pc name\printer name. That's the easiest way to do it. Of course, if your laptop is not also running XP, be ready with the Vista (or whatever) drivers. I run 5 pc's on my home network, and in order to do seamless file and printer sharing, I have setup the same account name with the same (very strong) password on all the machines (xp pro, vists ultimate, windows7rc, home server) and I can access resources from wherever I happen to be at the time.
Good luck, let us know how you make out.
Best regards
The Cleaver