Windows media player

[codswallop]

I have been ploughing my way through Vista and I am now trying to burn cd to cd.As a tryout i have ripped a few bought cd's and dragged some over to compile a cd of my own with no problem BUT if i try to add some of my favs. from the harddrive it won't recognise them and therefore won't burn the whole selection.Can anyone explain this please??? Thanks!!!!