Please tell me how to resolve my slow computer response time

I'm having a hard time determining why my computer is running slow. It ran Norton Antivirus to identify if a virus has affected my computer and the results came back fine.
I performed a disk clean up and a disk defragment as well however, when multiple Internet sessions are open the computer continues to run slow.
I have also added additional memory to my hard drive as well.
Currently I am at a lost as to what I can do to expedite my PC slow response time.
Please advise.
