What he ^^ said, to be specific you have installed a "WGA" update whichs checks your machine's product key (License)

All Dells from circa 2004 onwards have this unique product key stored in the BIOS, that when reinstalled with the Dell Windows XP CD will retrieve the product key.

You will also see this printed on a label on the machine itself.

What version of XP do you have? If it's Professional then you will be out of luck as you will have to reinstall the OS.

It's most likely the machine came preinstalled with "Windows XP Home Edition", it will be printed on the aforementioned label.

Using a key from another machine is illegal, and may not work due to WPA! Plus it will only work on the correct media/edition, i.e. XP Pro retail is different to XP Pro OEM.

Best thing to do is to try and change the product key with the correct one printed on the laptop, don't need any third party software, can be done OOBE.

Martin