[Alpha]

Seconding what Espy said to have a complete backup before you format it.

On the note of what might be wrong....could be a few different things. My best guess would be that there is some sort of virus/malware on the computer or that the issue lies in the processor/RAM. (not an expert, just have tinkered a bit)

When you say that the sound is "choppy" and it makes the videos lag, do you mean that the video skips or is slow, or that the sound and video don't sync up?