Its supposed to do that. You will have to erase it and sync it to work on your desktop. It will work with your desktop then. BUT, when you get ready to use it back on your laptop itunes will ask you again to erase and sync again. If you are charging only, click cancel. The song will still stay on the device. Keep in mind when you sync it doe NOT erase the songs or files on the laptop OR the computer. It only deletes the short cuts to the songs on the Ipod or Iphone.
Either way, if you have the songs on a file on BOTH the laptop and Computer you are good to go. If you do not have access to both the desktop computer or laptop at the same time I would not do it. Example. Lets say you have files on your Iphone or Ipod device that came from your laptop. When you hook the device up to the computer it will prompt you if you want to erase the files. You have to make a decision at that point. If you are done with listening to the songs that are currently on the device taken from the laptop and want to hear some songs that are on the desktop computer, then go ahead and erase and sync. If you want it to have the same songs on both laptop and computer, you will need to back up your library and use that same back up file on both machines. ALSO will need a copy of the same songs on BOTH computers for it to work correctly. Hope this makes sense.