error 80048869 - help Jonathan !

MSN Messenger started giving 80048869 errors a few weeks ago, for all users
on our Windows XP Home PC. Was running 7.5.0299 version. The built in
troubleshooting tool gives a "Your machine appears to have an invalid IP
address" error, and pressing the Repair button does not fix the problem. The
Service Status button shows the .NET Messenger service is up and running.
I've gone thru, a few times, Jonathan Kay's published 9-step
troubleshooting procedure, right through to re-registering certain .dll files.
I've tried various configurations of Windows, Trend-PC-cilllin, and our
DLink router firewall settings - everything from disabling all firewalls to
setting warnings when MSN Messenger tries to access the internet. The
firewalls catch and display the warnings OK, when so configured, so I believe
they are not the problem source.
We can successfully connect thru both webmessenger.msn.com and through
Windows Messenger.
I've uninstalled 7.5.0299 and installed 7.5.0324 and the problem persists.
I'm considering 'downgrading' to a version 7.0 release intended for Windows
98/ME. Browsed somewhere and someone recommended doing this.
Does anyone have other suggestions to assist in resolving this problem ?
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Thanks...brasha