The default for the guest password in XP Home and Professional is BLANK i.e there is no password.
To apply a Password to the guest account follow this procedure
Please log on to this computer with an Administrator account and turn on the guest account.
Click Start->Run->Type cmd and click OK.
Input the following command and press Enter.
Net user guest password
Go to Control Panel->User Accounts. Click the Guest account and now you can change the password.