how to strip down the default Xubuntu (or Ubuntu) distribution?

Rather than stripping down the system, perhaps consider installing a basic system with a "minimal cd" and then build your ideal environment up from there?

See here for the ISOs: https://help.ubuntu.com/community/Installation/MinimalCD

To actually answer your question though, a friend of mine wrote up this document http://www.stevey.eu/2008/09/diet-ubuntu-the-in-depth-guide/

While it is focused on stripping down the Ubuntu desktop, many of the packages are the same on Xubuntu.

How about using the Slackware distro to build up an OS tailored to your needs and to your machine? Perhaps take a short-cut and use Wolvix (Slackware based) as a good starting point and then just add in any additional modules (or just some packages) if you feel the need. Wolvix is not exactly bare-bones but it's a lot tinier than you're going to get even Xubuntu to be.
http://distrowatch.com/table.php?distribution=wolvix

Good luck and regards from
Tom :)

If you are still having trouble with this then please post it as a new question. Only the most recent questions tend to get looked at so posting/reposting a question just before america arrives online after work/school gives the best chance of getting a good few answers.
https://answers.launchpad.net/ubuntu/+addquestion

If the problem has been resolved then please follow the link to the forum thread and mark it as Solved.

Good luck and many regards from
Tom :)