Create a new profile & install ActiveX support (Windows media player) for Youtube from here
For Yahoo mail best set the user Agent string ( via privacy bar or Prefernces Privacy ) to Firefox.
AFAIK You have tempered with some default settings - this gives You more privacy and less features - nothing wrong really except that it is just not Your day.
The new profile should normaly give You default settings which are good for most things. If not? Maybe You broke your current K-Meleon install.
Backup private data, reinstall and create a new profile.