|What key are you using? Is it the one that is on the bottom of the laptop? |
If it is you will have to activate by Phone if you used a non dell XP CD to install the fresh copy of XP.
What was the original XP version (Home/Pro)?
What CD did you use to install XP? Where did it come from?
Was it supplied with your laptop?
Does the laptop have a recovery partition?
Most new ones do unless you formatted it away.
You need a Dell OEM Install CD for your specific laptop as it will have an OEM key built in and will be SLP BIOS locked to your laptop.
Answers to the above questions may be more helpful in guiding you.
☺ When everything else fails, read the instructions.