Sounds to me like a connection problem with the USB port. You just need to use the process of elimination to figure out if it's your computer, your iPod touch, or the USB cord.
If you follow these questions you'll figure out where the problem is coming from:
-Have you tried using a different USB port on your computer (as someone else mentioned)?
-Do you have another USB cord you can use? If so, try it and see if it works then.
-Do you have any other USB peripherals (e.g. printer, iPod, phone, mouse) you can hook up to the computer? If you do, hook it up --if it works then it's not your computer causing the trouble.
Once you figure out the source of the problem, post a reply and we can go from there.