itunes wont let me drag and drop music to my iPod touch

We had to restart my computer recently and now when ever I connect my iPod touch to iTunes it says that it has been synced with another library. So now for some reason iTunes won't let me drag and drop music to my iPod touch. I've selected "Manually manage music and videos" but that doesn't do anything. I go back into the music tab and click "sync music" but a box comes up and says "are you sure you want to sync music? All existing content on the iPod 'Pegasus' will be removed and replaced with songs and playlists from your iTunes library." and I don't want my iPod to be replaced with what's in my iTunes library because it only has the songs that I have purchased. and that isn't many...

So what can I do so then iTunes will let me drag and drop music from my iTunes library to my iPod touch ?