Impossible to reinstall the combo update 10.5.2

Source: XLR8yourMac

If a Mac OS X crashes with say a kernel panic ... often times the best thing to do is reinstall the latest Combo update. It will overwrite files and repeat stages of maintenance. In most cases, the problems will be solved.
Remember, however, that if you have the latest updates Airport and Time Machine, this lifeline will not be allowed.
In fact, these updates change the version number of Mac OS X, preventing combo update from working. Apple need to change these updates or release a new 10.5.3 which will allow this reinstall.
We take this opportunity to inform you that the latest Security Update we are talking about causes serious instability in some people's machines. We advise you not to install it while waiting to get a clearer picture.
We are also advising those who can not do without their machine for the time of a total reinstallation of MAC OS X to think about cloning their system. This solution is the most convenient for a very quick restoration in case of problems of this kind.
Do the clone before upgrading which will ensure a very simple backward migration, even in the event of major bug.