|I don't use windows media player so I can't say for sure. This might help:|
If it's not WMP, what are you using to view them?
As far as the youtube videos, if you're viewing them with IE there is a zoom feature. With an IE page open (on my old IE8) it's just above the windows taskbar on the bottom of the screen. I imagine other browsers have similar abilities.
However, the browser zoom would affect all pages, not just youtube, so that might not be the problem. And as I mentioned above, it may be a video setting too.
If you can't get it figured out and others don't post in with a solution you might check to see if you have a restore point from just before the problem started. If so you can restore to that date and likely fix the problem as I'm sure it's a setting somewhere.