I have Windows Media Player 10.

This is how you can make Windows Media Player (WMP) your default player:

Start WMP.

Click on tools, then options. (If you are in full view, you have to click on the tiny arrow in the upper right hand corner. It will say access application menus if you hover the cursor over it.)

Select the 'file types' tab.

Click on the files you want to play in WMP (Or just select all).

Click on 'Apply'

Click on 'OK'

Have fun!