Looks like all your known ducks are in a row:
1.Win XP's firewall is taken care of.
2.Windows UPnP seems to be working ok.
3.No software firewall interfering.
4.No NAT-based router interfering.
5.Your computer's ip matched the internet ip as seen by other bittorrent-running computers on the same torrent.
Which leaves as possibilities:
1.Virus and trojan activity, which acts as firewalls and proxies -- secretly rerouting data through themselves to "read" or "corrupt" that data in some way.
2.Your ISP is firewalling you in some way.
Try SHIELDS UP! at
http://grc.com
DON'T just check bittorrent ports!
The goal is to see if you can get open or at least closed results on ANY ip port, then worry about why/why not.