Sound and Video Problems

I'm a novice in using the messenger technology and I am having big problems
with the communication (total distorted, scattered audio - can't recognize
anything and absolutely slow video - only changing images) using following
equipment:
- MSN Messenger 7.5 on both sides
- 2 x Labtec notebook webcam with built-in microphone
- DSL Internet connection with 512 connection speed on both sides
- Windows XP SP2 on both sides
Everything is installed ok, no hardware conflicts.
Can someone advice me, where the problem can be?

Thanks, Robert