There are several ways to do this, depending on what you've done or have.
One method is to just rerun setup. This only works if its the same computer.
If you're planning on upgrading the motherboard or something then log in as admin, insert your Xp cd, let autorun bring up setup (or run it yourself) and select upgrade and follow the prompts for the installation type of your choice (custom, typical...). When your computer restarts turn it off when you see the bios part of the bootup. Replace the mb and then turn on the computer again and let setup finish.
Another is to use the File and Settings transfer wizard to export just about everything and then import it back into your system after you've reinstalled. This does not install programs, just their setting so be sure to reinstall the programs you want first.
This can be done to an external harddrive, flashdrive, cd/dvd burner, or even directly to the new computer if its setup and running on the network. It quite a useful tool if you do a little research on what it will and won't transfer. I use this tool quite often when doing hardware upgrades.
a guide to reistalling Xp Home I found on MS.