Try some of the following:
1. Restarting the computer and replugging the iPod in
2. Plug the iPod in with iTunes opened, then 'restart' the iPod (press Menu + Select key)
3. Find another Windows computer with iTunes and see if your iPod can be read
4. Find a Macbook and reformat your iPod with it before plugging it into your Windows again
If the Macbook can read your iPod but your Windows can't, it's probably either the computer's problem, or the latest version of iTunes. If both Macbook and Windows can't read it, I suggest you go back to the shop and ask for a refund.