Vista - Automatic Volume Fluctuation Problem

Good morning,
The reason I post this thread is because I am runing out of idea and I would
need some clue. I have instaled the version 9 windows live messenger. My
system is composed of XP Sp2 , with a creative Xfi Pro Sound Card, and for
the cam a logitech of which i have disabled the mic. All my driver are up to
date.
PROBLEM DESCRIPTION:
When I do a audio webcam set up, or start a conversation, the volume of my
center speaker slide automatically to almost 0. If I try to restore to
default via the sound properties, it just slide back to almost 0.
PROBLEM COMPARAISON
When flying online with Vatsim network and FSDT software i do not meet this
problem. Neither do I on playing online on EA server. On starting video
editing program I do not meet this behavior as well.
FIXES I TRIED
Unistall all sound driver and software.
Reinstall and update
Research on internet at Creative site and others. Some people seem to meet
the same strange behavioour.

I would really appreciate some feedback please,
In advance thank you