Open up the Scheduled Tasks tool from the Start menu, either via Programs > Accessories > System Tools …
… or via Settings > Control Panel > Scheduled Tasks.
It’s likely that some tasks have already been automatically set up, either by Windows or applications that you’ve installed.
Double-click Add Scheduled Task, to launch the Scheduled Task Wizard.
Click Next, scroll down the default list of programs and highlight Disk Cleanup.
Select the frequency …
… and the time you want the program to run.
Finally, enter your username and password.
Click Finish …
… and Disk Cleanup will now be appended to the list of scheduled tasks