SYNCING MY NEW SONGS WITHOUT ERASING MY DATA AND SONGS

So here is the deal my pc have some issues during some months and is where i synch my music with my ipod touch, ipad and also ipod. Then i send it to service to format it and then my pc was new. And when i have already purchased some songs from the store and when i tried to sync again my devices (ipod, ipad and ipod touch) I could but it asked me if i want it to sync and erase all my songs and data from my ipod in which i didnt want because i have around 100 songs wich i have already bought and downloaded in that same pc. So how can i sync my ipod, etc with another pc in this case with mine without deleting my data and songs?

Note: I have Windows 7