One of the biggest directory in a Windows 7 (and not only) operation system is the winsxs.
What is it?
Winsxs, which stands for ‘Windows Side By Side’, is Windows native assembly cache. Libraries which are being by multiple applications are stored there. This feature was first introduced, in Windows ME and was considered as Microsoft’s solution to the so-called ‘dll hell’ issues that plagued Windows 9x. 
In this post I will share a solution in order to reduce its size
Actually, on my one year old Windows 7, it is 11 GB (about the 5% of the total used space)
Looking on the Microsoft Support I found this interesting command line utility DISM.
Execute the command prompt (Start->Run…->cmd) as administrator and type the following statement:
dism /online /cleanup-image /spsuperseded
The tool takes a little to perform all the operations
Once the tool finished you can check you directory, I gained 4GB.