MSN Messenger + Windows XP + Service Pack

Dear all,

I found a situation, and I have no idea to resolve it.
Could you give me some suggestions? Thank you.

1). There are two subnets behind my home router, they
are 192.168.2.0/24 and 192.168.1.0/24.
Before I upgrade to service pack 1 or 2, MSN messenger
6.2 can chat with video and audio very well.
(LAN: 192.168.1.0/24)---[ home ]-------Internet
(LAN: 192.168.2.0/24)---[ router ]

2). After I upgrade to service pack 1 or 2, the two LAN
side pc cannot use video and audio chat.

3). I found the content of first UDP:Invite packet is
filled with the WAN IP address of home router after I
upgrade to service pack 1 or 2. In other word, before
upgrade to service pack 1 or 2, the content of the first
UDP:Invite packet is filled with the IP address of two
LAN side PCs.

4). I think this is UPnP and Service Pack issue, because
if I disable the UPnP of home router, the video/audio chat
is ok. And it also ok if we enable the UPnP of home router,
and we use the old Windows XP version(no service pack).

Best Regards,
Steven Yu