Really sorry - my mistake. Parential controlls CANNOT be applied to the guest account, so you should create a new limited account, you can still name it guest if you want. From your account you can forbid it to do whatever you please. You can even limit the account to be able to only surf the net.
BTW are you SURE that the guest account can do all you mentioned above??? Try to change some system preference via the guest account. As soon as you try that, you should get a UAC popup that will allow the guest user to change a system preference of an administrator ONLY if he already knows and enters the administrator password. Plus, the guest account might be able to change things in the control panel, BUT ONLY FOR ITSELF. This means the guest account has it's own system preferences that everyone can change (since it's not password protected), but no one (accept you lol) will be able to change the setting for your account.
Here's an example:
The people using the guest account wants to turn off ClearType, to make the fonts look sharper. So it can do that, but the ClearType feature will still be on on your account. That's kinda what accounts are for... So that everyone can have his/her own preferences...