It may be that you've installed something that changed your default music player. Go to My Computer/Tools/Folder Options/File Types and see what programme CDA, MP3 are associated wth. (select them and it show in the bottom of the window).
If you select "Change" it will offer you recommended programmes from the ones you have installed - Windows Media Player etc. Select one and see if the quality improves, if it does, change the other music file types to the same one.
If the quality doesn't change, let us know and we'll try something else.