If you purchase the songs, then I think there's a permission issue at hand. AFAIK Unless I'm mistaken, open iTunes & go to the Advanced menu, then select De-Authorize computer. When you purchase songs, I think you're only licencing them to be able to downld & play on your computer. But I could be wrong. However I had thought you were allowed to make at least one copy of them or something. If you want to know the exact answer, post on the iTunes or iPod forum.
iPod Forum