Sometimes a reinstall of the operating system is the easiest way to tackle multiple or nonsuperficial problems with computers, since doing this will refresh the underlying operating system software and hopefully clear any corruption and configurations that may be causing the issues.
Apple has progressively made the process of reinstalling easier, to the point where now it is practically seamless to pop in the Snow Leopard installation DVD and run a reinstallation, then perform an autoupdate and be right back where you were, maybe. In some instances, after performing reinstallations people may find certain applications will not work anymore.
Usually those that are broken are complex ones that have installed resources throughout the operating system, especially in hidden system directories. Such applications are usually large productivity suites like Adobe's Creative Suite, Microsoft Office, and technical applications for science and engineering like Matlab and LabView (these are just examples, and plenty fit into this category). If you need to reinstall your operating system, be sure to know what applications you use regularly and have an easy route to reinstalling and reconfiguring them. Sometimes fixing problems with them is as easy as setting file paths, but other times you may need to reinstall shared libraries and resources, even if they are just fonts.
Beyond third-party applications, there are times when Apple's supplied applications will not function after reinstalling the OS. This usually happens when people have tried reinstalling without fully running the OS X installer, and instead have tried restoring from Time Machine backups; however, it can happen if people try to migrate applications over from a backup after performing a full reinstallation of the OS. Many of Apple's applications are version-specific to the OS, so if you restore one to the system from a backup of a different version of the OS, you may get a version mismatch error such as "You have Mail version 4.2 (1078/1081). It can't be used on Mac OS X Version 10.6.4 (Build 10F569)."
If such an error occurs, usually you can fix it by running the latest Combo updater for OS X, which will update all the components of Apple-supplied applications to their latest versions; however, sometimes this may not work if the current application on disk is corrupted. Therefore, besides reinstalling again to get a working copy of the application installed, another route is to use Pacifist to manually install just the needed application from the OS X installation DVD and then run the combo updater.