Invalid Product Key I got from Dell...

What product key are you speaking of?  The one on the side of the computer should work for every OS reinstall on your machine.

It's really too late to call Dell right now but will Dell give me another XP Installation CD if I give them mine seeing that it's useless now? I bought it from them and it doesn't work, and I don't think it would be fair if I had to pay for another one.

Getting another CD would not fix your problem since you would not get another product key with it. The one on the side of the computer is yours for all reinstallations. How do you know it isn't working? Have you tried to reinstall windows with it?

If the XP OS was DELL installed then the Product Key usually on Computer or Tower only works with a DELL reinstallation XP CD won't work with a retail Microsoft XP CD.
If you boot from reinstallation XP CD it usually doesn't request a Product Key , sometimes does though.
If you use from Within Windows (Install > Upgrade > New Installation) it always requires a Product Key.

Also a retail Microsoft XP CD Product Key probably won't work with a DELL reinstallation XP CD.
(haven't tried the above so not sure ?)

Oh, it was on the side of my computer... I was using the product key from an envelope...

It worked. Thanks a lot, dude. :)