Have you checked suggestions here http://www.gnutellaforums.com/connec...t-connect.html
Make sure both LW's are not attempting to connect using same port. Your router will only allow one program to use a particular port.
I'm wondering if your router is only giving UPnP for your Vista machine & not the desktop. Is your XP system up to date?

Be interesting if you tried the above suggestions after Rebooting your computer whilst not using the laptop's LW at all. This will free up all the ports & UPnP your router is offering. Either reboot main computer router is connected through, else reboot the router itself (simply turn off for a minute.)