As Mick wrote, it would seem that your Vista DVD is for an upgrade install.
If your XP disk is a full install disk, rather than an upgrade CD, you can
either format the HD or delete the HD's partitions and do a full XP install.
Then, use the upgrade Vista DVD to install that system as an upgrade or a
clean install in a dual-boot arrangement. Tutorials for both scenarios are
available from various web sited. Do a Google search.
However, this brings up back to the problem you state in your original post
about not being able to format using the XP CD.
This might be useful
"ronscook" <firstname.lastname@example.org> wrote in message
> Thank you for your support. I think the problem seems to be the product
> When I enter my product key I get a message "To use the product key you
> entered start the installation from your excisting version of windows" I
> had this O/S for longer than 3 months so phoneing for help was useless. My
> computer has almost used all it's memory and I would like to format it,
> help appreciated
> "ronscook" wrote:
>> I have tried to format my computer, first I tried by just inserting the
>> no luck, then I tried booting it from the disc, no luck then I tried
>> into computer management right clicking my c drive but the format option
>> no available. I have evn tried phoneing Microsoft but they want $80.00.
>> Finally I thought I would format it by using my XP disc, installing XP
>> then upgrading to put vista back on the system but it won't let me do
>> either. I desperately want to format my computer can anyone please help