why does my iTunes window freeze and wont close until i shut my computer down, when the "iTunes is not the default media player" message appears? and how do i fix this? i just want to fix my music, but it freezes and i can't use my computer correctly.

when i plug my iPod into my computer, iTunes opens as normal, but for a while it's been giving me a message that says "iTunes is not the default media player" and after i 'tried' to do that, i'll close the settings window and my iTunes window will freeze and not close, reload, or anything. and if i "open drvice to view files" that window will freeze too, the internal storage window. i can't get iTunes to close with ALT+F4, or right clicking the tab on the bottom of the screen to close it, or clicking the 'X' button. the only way it closes is when i shut down my computer. please help me fix this.😕 Its getting Really Really tiring having to restart my computer over and over. 😟