You can always re-format and go back to XP, I believe you have simply made an error during the installation process.
If the CD is an OEM copy for Compaq computers then you should have no real issue as the OEM key is linked to the bios. If the CD is for another computer and is not an OEM copy for Compaq computers then you will need an XP key to activate it.
I bot Compaq Presario c502 us with Vista installed this Friday, but all of my applications are not compatible on Vista, so I delete all of partition on HD, and format, then I reinstall my XP ( not coming with this laptop, but I reinstall my another pc successfully) by XP home installation CD. It gave me a big blue screen and terminates installation!
I heard a rumor about some new pc with vista won't allow to reinstall another OS, may have to talk with support to change some BIOS setting!!
I have no any idea right now, stuck, I need this one badly.
Any comment welcome,
Thanks

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