Finally, Windows directions for allowing iTunes music sharing through XP's firewall:

Savvy Windows XP users will have enabled the built-in firewall for their network ports. This works well to keep out the bad guys, but it also stops incoming traffic you might actually prefer to allow.

In order to allow iTunes sharing with the firewall enabled, you need to 'punch a hole', so to speak. Well, the truth is you actually need to punch two holes. You need to allow the following two ports to get through if you want to access other peoples' iTunes shared files:

Port 3689 TCP
Port 5353 UDP

I still get relatively choppy performance, but at least the firewall's not the issue now.