I found a really easy way to do it.
1, load all of the tunes you have in Itunes onto your Ipod.
2, connect your Ipod to your new PC and wait for Itunes to recognise it.
3, come out of Itunes, go to my documents, and create a new folder to put you music into.
4, go into my computer, click on the Ipod folder, then copy all of the tunes into the new folder you have created, this can take a while, depending on how many there are.
5, disconnect your ipod.
6, go into Itunes- File- Import folder. Import the tunes you have just copied into Itunes.
Job done.