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What is the winsxs directory and why is it so large?

The %windir%\winsxs directory in Windows Vista and Server 2008 is initially large, and can grow enormously.

Win SXS - "Windows Side by Side" - keeps multiple copies of DLL's, including old versions, in order to let multiple applications run without any compatibility problem. The feature was first introduced in Windows ME as the solution to the "DLL hell" that plagued Win9x.

The best technical description can be found on TechNet - an interesting read that talks about how Vista and 2008 now store OS information in "components" - which are kept in %windir%\winsxs. This is the central "component store" for all DLLs and is in fact the only place where they are kept - all files in other locations are actually "projected" by hard linking to the component store. This replaces the use of "INF" to describe the OS.

After reading this it should be obvious that this folder should not be "cleaned" through any direct process - in fact there are various protections in place to stop you from trying to do this - the only way is to uninstall any unused software (even this may leave some duplicate files around but that can't be avoided).

More info: The Secret of Vista's Winsxs folder