Problems like this are very difficult to diagnose over the internet unless it's happened to a million people. I haven't come across this problem before.
It can only be a few things wrong.
1. Yours windows system is unstable either software or hardware
2. The media you are using is corrupt
3. Sony Vegas as an undiscovered bug or is corrupt
4. Some other 3rd party software is malfunctioning
5. User error or some other fault.
I know this are very general but might give you a heads up into what direction to follow ie if your system is stable then move onto the next one. Does the media play ok otherwise is so go to number 3 and so on.
It's rubbish help I know but the best I can do.