Windows Vista user account password forgotten?

Hi, My daughter has forgotten her Windows Vista Home Basic user account password. She can use her laptop via another user account but cannot use defrag or install 3rd party software since it asks for the password we dont know. The laptop is getting slower and slower obviously requiring a defrag and other maintenance which we cannot do. We have no need to get back into the old user account but just need to be allowed to do maintenance on the system. Can someone please tell me how to resolve this problem.
Best regards, Peter Sullivan.