Compaq Presario SR1740CF-ER938AA Pro...

You need to call to a number in your country or enter to the web page when the serial number is asked when your are installing windows, you shold see an option to activate your copy of windows, you call or enter the webpage, wirte the number in your PC and then you'll get a valid serial number for your original windows XP; you'll have the option to register your copy of windows, but it's not necessary.

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Is that the original cd came off on the box when you buy it. If yes it should be ok.. If not the the cd is wrong..there is no way you can use the cd key anymore. you need a new OE with cd key.