After reading Joseph Doyle's submission regarding this I would like to point out that you can set a Guest account password by simply doing this. This method is somewhat simpler in that you do not have to mess around with CMD.exe.
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.
I have always considered security a top priority and considering the ease of doing this I would highly recommend that everyone sets a Guest account password. Even though the Guest account is disabled by default, why not do this just for the extra bit of security?
Local Users and Groups is not available in XP Home. A way to set a password for the guest account in Home and Pro: click start - run - type: cmd - in the command window type: net user guest * - hit Enter - you will be prompted for the password to use.