|Thanks for coming back with more information. Now that we've eliminated background processes and buffering as the problems, let's try to pin down the actual cause.|
Has this always been a problem, or did it start doing it recently?
Have you tried the DirectX diagnostics (Start>Run, type DXDIAG, click OK)?
What video chipset do you have in the laptop?
Try rolling back the video card driver (do this from device manager) to the original one. Windows Automatic Updates may be updating the driver (Never accept any hardware drivers from Microsoft, they are often as buggy as heck). You might need to change Automatic Updates settings to a more interactive selection, otherwise it could keep happening. If you want an updated driver, download it from the laptop manufacturers website.
When you play a video, try opening Task Manager, and on the processes tab, take note of which process is using the most CPU cycles, and which has the most memory usage. This might provide a hint as to whether or not it's a software problem, and what program/process is the offender.
You could try going into the video card settings, looking for the Hardware Acceleration slider. Try lowering the acceleration. Try the same for the audio hardware acceleration.
Download Memtest, burn to a CD (should be bootable), boot from it and run a full set of tests. With luck, this'll rule out any physical memory (RAM) problems.
Have you scanned for viruses/spyware/trojans? I doubt this is your problem, but if we can eliminate it early, it's one less suspect to be looking at.
Does it make any difference if you change the video size (50% or 25%)?
Please let us know if you found someone's advice to be helpful.