can't connect to my FTP srever after installing SP2 - win XP pro

I have an FTP server on my win XP pro.
since I have upgraded mt XP pro with SP2, people from the internet can't
connect to my FTP server, only people on the intranet.
I have a router with ADSL connection, and NAT.
when someone from the intranet is connecting to the FTP server everything is
ok.
When someone from the internet is trying to connect to the FTP server he
doesn't get any response.
I can see on the router that the NAT is transfering the request on port 21
to the PC.
and I can see (in the windown firewall log) that a connection to port 21 is
made, but the user that is trying to connect to the PC gets time-out massage.
and on the "Computer Management / IIS / FTP_Sites / Default FTP server /
Properties / current Sessions" I don't see anyone that is trying to connect.

Does anyone knows this problem / knows how to solve it ?
Regards

Yossef