Put the XP CD in your CD drive
Hit F12 (or whatever it says to hit on the Dell boot up screen to get to boot order)
Select Boot from CD from the option list
XP install will lead you through several screens... the important one is to do a clean install (format your drive and install a clean image over it)
I would recommend formatting your disk as NTFS instead of Fat32 - it gives you more security at the file level but that is a matter of preference...
Once you re-format your drive and go through all the menus you wait for XP to finish installing. Then it will boot you into windows...
This is what we mean by installing drivers... you will notice your screen looks funny and you can't access your network card (among other things)
This is because you have not installed the drivers for these devices.
You will need to insert the cd that came with your computer that says "device drivers" on it. It has a graphical window that pops up and guides you through choosing the correct drivers for you laptop type and OS (which is windows XP in your case)
You will need to install drivers for your sound card, wireless network, wired or ethernet network, video card, touchpad, and some intel chipset drivers
Overall it is a pretty easy process and much of the way is explained in easy steps by the setup programs. It is a good exercise but be warned that if not done correctly your system may do odd things later so it might be best to get someone who has done it before or research a little more till you are confident that you are doing it correctly.
Also be warned that you are formatting your hard drive and will lose ALL data on it so if there is something you want to keep, BACK IT UP FIRST!
hope that helps