SIP no longer working

Windows XP Pro SP1 + latest updates etc.
Running Windows Messenger 5.0 RTC client for SIP in a
corporate environment

For some reason, my SIP access has gone away. I tried
everything from reinstalling the software to reinstalling
SP1. I've tried manually removing the MSN Messenger that
comes with Windows XP and I manually removed RTC client
with "misexec.exe /x messenger.msi"

Exchange component was switched off here, and .NET is
working fine. However, when I try to Sign In to SIP, I
get the following box:

Title: Windows Messenger
Body: Signing in to SIP Communications Service failed
because the service is temporarily unavailable. Please
try again later.

I had the Exchange guy check out the SIP account on server
side and it checks. I can connect on another PC, so it is
local. Does anyone has any suggestions?

Randy

Hi
Did anyone get back to you on this

Thank