After allowing a friend to use his microsoft windows 7 computer, a user notices that some applications are not working properly. Running an antivirus scan returns no reports of infection. What should the user do to restore the application to working order without risking the loss of data?

a. Use task manager to restart each application that is not functioning properly

b. Use windows emergency repair disk to repair the operating system

c. use the control panel to remove each application that is malfunctioning

d. use a system restore point from a date prior to allowing the friend to use the PC