I think, some of you have faced this problem and always I am asked why this folder is too large. let me tell what is this folder?
This folder comprises Windows patches. If Automatic windows update option is enable, this folder will be grown. Therefore you can disable that option or delete files which are already installed. Actually you cannot understand which one is installed, which one not. Best thing is delete all files under download folder.
1- create a new text file on desktop.
2- open it and copy this code
net stop wuauserv
ren Download Download.old
net start wuauserv
3- save it and close
4- rename file to rename.bat
5- double click on it.
After the service has been started again, a new download folder will be created again. This will give you a change to delete the folder named download.old