http://docs.info.apple.com/article.html?artnum=106549
This article states that you cannot reinstall OS X 10.1 over an existing installation if the security update has been installed. Instead you must reformat the hard drive before reinstalling OS X 10.1! There's a reader report on MacFixIt that claims success in reinstalling 5G64 (the pre-security-fix build) over 5L14, but it's unconfirmed by anyone else at this point.
I have to agree with the majority here - this is amazingly bad. It used to be that an easy way to fix an unfixable OS X problem was to re-run the installer; it would repair the existing installation without touching anything. Now, instead, it appears we have to reformat our drives in order to reinstall. I, for one, would not be happy about having to backup and restore some of the more 'deeply rooted' programs I've installed such as mySQL and SAMBA. So instead of a quick "install in place", I'd be looking at the time loss from not only backing up and restoring my normal user and applications folders, but also reinstalling and configuring all of my more complex UNIX installations ... some of which took considerable time to get working.
Apple needs to fix this problem, and quickly!

