The parallel installation on my desktop is configured as I have described in XP Unleashed, but since this was to be a new installation, rather than reconfiguring an existing installation, I used nLite to do most of the heavy lifting and then burned an installation CD. The laptop parallel installation is a different story. Again I used nLite, but I trimmed out a good deal of the Windows components that I wouldn't need. Using it basically as a tool, not a primary OS, it can have a much smaller footprint. I put the Documents and Settings folder on a small separate partition as well. On my laptop I also edited the registry to point to the primary XP installation's existing Program Files folder on its own separate partition, and did concurrent installations of some of the tools and programs already installed in that folder to make them accessible from the parallel installation.
A parallel installation will boot a little differently; you will be presented with a boot options menu allowing you to select which XP installation you want to boot up. One installation will become the default, and will automatically boot after a timeout. You may need to edit your boot.ini file to get the sequence like you want it. On my laptop I'm presented with a choice of "XP Pro", "XP Lite", or "XP Repair Console", with XP Pro being the default. I use BootIt NG as a boot manager on my desktop, since I have a parallel installation on a second physical hard drive.
A parallel installation is a very powerful tool, and its use must be carefully planned, and exercised with extreme caution. If you have, for example, a file somewhere in your primary XP installation that you can't delete no matter what you try, you can reboot to the parallel installation, navigate to the offending file and delete it without difficulty. You just have to be completely certain that you are deleting the correct file.