1. Make sure you are logged on as an Administrator.
2. Go to Start > Control Panel > Administrative Tools > Computer Management.
3. Select the "Users" folder under "Local Users and Groups".
4. Right click on the "Guest" account and click "Set Password". When a dialog comes up warning you of the possible consequences click "Proceed". You will then be given a dialog that lets you set a new password.
A worthy addition to your computers security especially if the guest account is enabled. Any password is better than none.