it is really quite simple...the installation ID which removes the 30 trial is generated form the product key and a hardware identifier.
since you already installed xp on the machine, activation can occur BUT it will register the product key of the initial install. you cannot use the product key on the CD you just bought to activate the expired version already installed.
you have 2 choices:
1) reinstall xp using the OEM cd you purchased.
2) activate the expired install currently on the drive