Incoming connections blocked by WinXP?

This problem has already cost me the better part of the weekend, and I am getting desperate.
The problem started last week after I installed Office 2003. After that I noticed that active ftp would not work anymore. I had no time to look into the issue, so I switched to passive ftp. No problems.

On Friday I wanted to play my favorite game online (as every Friday, without ever any problem), but I couldn't join any session. I fiddled around with my router / firewall, but all seemed fine.

It seems my computer is not accepting any incoming connections anymore. Of course I have the build-in XP firewall disabled, and the external router / firewall cannot be the problem (see below).

On Saturday I had the suspicion that it might had to do something with Office 2003, and did a system revovery to the day before I installed Office 2003. Everything worked fine. Unfortunately I decided to track down the problem manually, and undid the system recovery.
Murphy's law kicked into action and things went bad: There wasn't enough harddisk space for the recovery, it got stuck in between and to make a long story short - all recovery points are lost. So I am stuck with my current config.

What I have done so far (with no success):
Uninstall and install SP2
Uninstall and install Office 2003
Installed WinXp over the existing installation: Here I noticed some very strange behaviour: After installation was nearly done, it complained about two .dll's being faulty (one was named inet-someting). Until I installed SP2 I could not get any network connections to work. (Uninstalling SP2 has the same effect.)

Any help would be welcome.