How to remove Windows 7 system.OLD dynamic file

Found that the space of the C drive installed a Windows7 and actually took up dozens of G, what is going on? Look carefully, found a WINDOWS.OLD folder under the C drive, huge! What is this Dongdong? Looked inside the Dongdong, found that when the installation of windows7, the backup directory of the previous operating system!
This directory is special, can not directly delete the WINDOWS.OLD folder, otherwise it will cause Explorer to crash. So how do you easily delete the WINDOWS.OLD folder?
1, find the cmd.exe file (in the “start –>programs–>attachments”), right click to select “ Run as administrator>
2, run command
rd/S/Qc:WINDOWS.OLD
or
rd/S/Q$WINDOWS.OLD
3, when prompted to confirm Delete the target WINDOWS.OLD folder, confirm the input y and press the Enter key.