Laptop Freezes When Playing Video?

Just recently my laptop has been freezing during video playback. The audio stutters or runs out of sync with the video, the video becomes grainy and jumping 3 or 4 seconds at a time. Just generally struggles to cope with the file. It varies as to how long this takes to kick in. Some times it's half way through a film, sometimes within a few seconds. Once it starts everything is painfully slow and I have to reboot. I've also noticed it takes much longer to reboot sometimes.

I checked in my programs and the Adobe Plug-in had an "X" over it so I uninstalled and then re-installed, thinking I'd found the issue, but it's still the same.

I've run AVG to look for viruses and also cleaned up the registry and so forth with other software but the problem remains. It only starts though when I run video files, this is what seems to start it.