When video lags behind audio this can be for a number of reasons:

A less common reason in the computer world today is that the computer hardware is not quite up to the task.

In some cases users have forgotten to install or update DirectX, which is required for video overlay on many graphics cards.
You can download DirectX here:
http://www.microsoft.com/windows/directx/default.aspx

In some cases users are using players that cannot output a video overlay to the graphics card properly (Windows Media Player).
The players to try as an alternative are: Zoom Player, BSPlayer & Media Player Classic.