This iPhone is synced with another iTunes library........do you want to erase this iPhone
You have to connect the iPhone to the machine you originally synced it with and check the manually manage music and videos option on the summary screen, in iTunes.



After that you'll be able to connect your iPhone to any iTunes library and drag and drop media straight on to the device, in the column on the left of iTunes.



You can also try to back up your iPhone on the new machine, by right clicking on the device in the left pane of iTunes and selecting backup.
Note: If you sync with the new machine, music will not transfer from your phone to the computer, and you will have the same problem if you try to perform a sync with the original machine!