Doing a Restore point on an Inspiron 660 desktop.

My husband tried to do a restore point on his Inspiron 660 desktop and he went through all the steps and then it told him that it didn't accept it.

What is the secret in doing the restore point for it to be accepted?

Could someone walk us through the steps here.

I would appreciate any assistance here so that he can get this done for the machine.

He has Windows 7 operating system-but, that should not make any difference.

Thank you,

CarolF1