What have you tried so far in terms of troubleshooting this issue? Are you plugging the iPod into a high powered USB 2.0 port on the back of your PC? Have you tried a different USB cable?
What happens if you try to reset the device with it still connected to the PC?
Since this iPod used to work with this PC, can you think of anything that might have changed with the PC since the last time you were able to successfully sync the iPod with iTunes? Perhaps you installed a new USB device such as a printer or external hard drive. Maybe you installed some new software or updated an existing piece of software?
Have you carefully worked through each and every single suggestion in this Apple support document?