Reset Snow Leopard user accounts/preference

My kids completely changed everything in their user account, what is the easiest way to reset everything to default (without reinstalling everything) including:

• Name of hard drive
• Menu items in Finder
• Dock size and items
• Size/appearance of things
• Even some applications have been moved from their original location

• Name of hard drive
  Click enter, rename it

• Menu items in Finder
  What do you mean by that?

• Dock size and items
  Trashing ~/Library/Preferences/com.apple.dock.plist will do the trick

• Size/appearance of things
  Trashing the .plist file under ~/Library/Preferences

• Even some applications have been moved from their original location
  You'll have to move them back to /Applications manually.

In order for that not to be reproduced, you could try Parental Controls under System Preferences > Account and limit what they can do on their account.

In addition to Loïc’s excellent answer

Menu items in Finder

What do you mean by that?

I believe he means the Finder’s Sidebar and such. In order to reset those, thrash the following file and logoff (or restart):

~/Library/Preferences/com.apple.sidebarlists.plist

Even some applications have been moved from their original location

You'll have to move them back to /Applications manually.

Yes, unless he has a Time Machine backup… which I believe he doesn’t. :)