Think you may need to continue to look at your computer to clean it out, you don't really have enough info here to let someone determine what it could be.
Some of the things that would be useful to know are -
What operating system you use
What type of connection you have
Checking to see if settings have changed
What resources the computer has
You say at the start that video's on web sites do not play but YouTube videos do, then further down you say posted YouTube ones don't play fully, do you mean YouTube videos posted on other sites don't play and only YouTube videos on YouTube play fully?
Personally I think you need to look at the cleaning up of the computer
Some of the things that would be useful to know are -
What operating system you use
What type of connection you have
Checking to see if settings have changed
What resources the computer has
You say at the start that video's on web sites do not play but YouTube videos do, then further down you say posted YouTube ones don't play fully, do you mean YouTube videos posted on other sites don't play and only YouTube videos on YouTube play fully?
Personally I think you need to look at the cleaning up of the computer