Short for Windows Side By Side, WinSxS is a folder in Windows XP, Vista, 7, and 8 that stores different copies of DLL and system files. Because programs can use the same DLL file, this folder stores the different files for compatibility reasons and for times older versions need to be restored.
The WinSxS folder is located at C:\Windows\WinSxS and it is not abnormal for the size of the folder to be several gigabytes. The size of the WinSxS folder varies between Windows versions because Vista and 7 do not have the i386 folder, which is where Windows XP stores cached DLL files.
The WinSxS folder and its contents should never be deleted manually. If you want to clean files from the computer, we suggest using Disk Cleanup tool.