iPod with iTunes is a one way street. You need some third party app to get your songs in your computer again.

Here is one Anapod Explorer™ iPod Software, iPod Backup, iTunes Alternative, iPod Transfer, iPod Manager, iPod Management, iPod Explorer, iPod Utility, iTunes Replacement, iPod Copy, iPod Freeware, iPod Managing, PC Software, Windows Software, Windows Vista Soft (can't guarantee this is free, whne you want iPod to PC transfer), but I'm sure there are others.