Plz Help : Latency in voice conversation with Messenger

Hi all,
it s a desesperate man who write these few lines. Let me describe
you my trouble
When i use Messenger for an audio conversation (with another msn with
same last version), after 15 sec of normal conversation it become
laggy. In fact almost 3 sec laggy. It seems that the problem comes
from my PC. When i speak, the micro level (the green bar on the lower
right side) raises 3 sec after i pronounced the word .
I have tested with the bad yahoo messenger and i have the same
behaviour

My PC :
Athlon 2600+
Asus NForce A7N266C * Nforce driver 3.13
1Go DDR PC 2700
ATI 9800 Catalyse 4.3
Philips Seismic Edge with lastest driver
SP1 and all windows update installed
Additional Info :
- I have configured well Audio and video param in Messenger option
- if I unmute Microphone in the control panel i have no delay to hear
my voice in my headphone
- Simple Windows Voice recoder show no delay
In fact it seems that it s only for messenger like soft...
My network connection : 128 Ko / s download , 256 Ko / s upload

Thanks in advance for your help
Patrice


-