First, plug your iPod into iTunes and in the main iPod screen, scroll down and select Enable Disk Mode (or Enable Disk Use). Then eject your iPod and plug it back in. Your iPod should now be in My Computer. Double-click on your iPod.
Then enable hidden folders.
First, go to Tools and then click Folder Options. Then click on the View tab, and under Hidden Files and Folders, click Show Hidden Files and Folders.
Then go to the hidden folder called iPod_Control, and in that folder, double click on the folder called Device. Once there, delete the file named "_volumelocked". Be careful. Don't delete anything else.
After you've done all that correctly, eject your iPod (using iTunes). Then restart your iPod by holding down the center and menu button until the screen shows an Apple logo.
Once your iPod has done this, check and make sure that it worked. Go into Settings, then Volume Limit. If it doesn't ask for a code, it worked! If not, have another go and see if it works.