The WinSxS (Windows Side-by-Side, located in %windir%\WinSxS) folder is a critical component of Windows Vista. It contains files that are necessary for adding or removal of programs, uninstalling components, and installing certain system updates. Unfortunately, the folder also has a tendency to swell in size, taking up large amounts of disk space (up to 6GB to 10GB or more).
Under no circumstances should you ever manually delete items from this folder. Doing so can interfere with critical system functions and may eventually lead to a situation in which you need to reinstall Windows Vista. However, one user has developed a tool that can safely remove unnecessary files from this directory, saving large amounts of hard drive space.
Christian Bering Boegh has created a tool called WinsxsLite, which works by deleting some unnecessary files and, in their place, linking to the latest versions of the same files (the WinSxS folder, by default, contains duplicates of many files). You can download the latest version of WinsxsLite (1.8.6) here. An instruction file is included–be sure to read it thoroughly before running the tool.
Note, however, the following stern warning: “WinsxsLite makes irreversible changes to the entire system partition. The only way to undo these changes, is to restore the system partition from a backup taken before running WinsxsLite.” In other words, unless you really need the 3GB or so that WinsxsLite will likely recover, you’re better off leaving the WinSxS folder alone.