Hello Daniel,
Chris offers some good comments. (I am always happy to see you post in, Chris. Thanks!)
If you have not already done so, ask you PC SP2 buddies to check their SP2 firewalls. Instructions on how to disable them are in the SP2 (James Weston's) section of the
the "... girlfriend ..." section of Ralph John's pages. (You may need to scroll down to see the SP2 info.)
Do you have your Mac OS X
b System Preference > Sharing > Firewall
enabled? If so, and if Chris' suggestions did not already get you working, try disabling your Mac OS X Firewall while testing with your PC SP2 buddies.
You were wise to test with several PCs to narrow down your problem to your PC Buddies that use XP 2. Have you done similar tests with Macs to determine whether you have any problems with one-way video to Macs?
EZ Jim