See here for more on CPU performance .
Most people add many utilities and tools and other neat-o software tools to their system over time.
The cumulative effect of these can be to consume a significant amount of system resources and memory, which slows down the machine. It can make a real difference, it's simple, and it's free. I recommend scanning your system for viruses on a regular basis. However, beware of virus hoaxes. There are chain letters that go around and around the Internet for years , talking about how if you read an email message you will get a virus.