can't start voice conversation

I'm using a brand new PC with XP home, MSN messenger 6.2.
The web connection with my peers work beautifully but I
can't connect the sound.
I'm using voice card of realtec AC97 (audio controller
INTEL ICH5) and the mic working good in the voice wizard
of the messenger program.
When I try to add mic to the conversation I got an error
massage that the other PC have problems and the
conversation ends.
My friends don't have any problems with other ppl only
with my PC.
I tried to uncheck the QoS packet Schedular but it didn't
help.
What should I do?
Thanks
Shmil