Change all folders such as my documents, my pictures, my videos, my music, recycle bin, desktop, public documents, public pictures, public videos and public music. Go to another disk, run disk cleanup, and select Delete everything except the latest restore point. All the necessary software except the system is installed on other disks, and all the software that has been installed on disk C and is not needed by the system is transferred. The tools for transferring software are found on the Internet. My system is 64-bit win7 Ultimate Edition. After one year's use, all C drives were kept clean, within 15G.