Remember, XP has its own firewall. So ensure it is either configured for LW or disabled totally.
Make sure your Linksys router is set for UPnP. You didn't specify the model, but here's a quick example of how to do Linksys UPnP setting (click on link) BTW if you have a linksys wrt54g, then manual port forwarding might be needed.

If you tried FW also & also your friend can't connect through the same isp then it sounds suspicious.

Try this program, bearshare lite & see if it connects. If not, then the problem may well be your isp after all http://www.filehippo.com/download_bearshare_lite/
BTW isp's will not normally tell you if they're blocking or filtering p2p.