A virus is a piece of code or program that is loaded onto your computer and runs without your knowledge and runs without your approval. Viruses do many things like replicating itself and deleting files. They use memory and system resources which causes computers to slow down. They also try to connect to other computers so they can spread themselves to other computer via networks and the internet.
Spyware is a whole different animal, it also is a piece of code or program loaded without users knowledge but, Spyware collects information about the user. It collects browsing habits, credit card numbers, email addresses, passwords or any knowledge that the writer wants to learn about the user. It can also interfere with user control of the computer in other ways, such as installing additional software and redirecting web browser activity. Spyware is known to change computer settings, resulting in slow connection speeds, different home pages, and/or loss of Internet or functionality of other programs. Corporations are paying big, big money to have experienced programmers to write code.

There is no one perfect Virus or Spyware scanner. You are required to install many programs to keep your computer and activity safe. You also need to pay attention when typing in web addresses. If you mis spell a name then you could go to a page and get Spyware/Virus infections.
Certain Viruses can be easy to remove while others can take several hours to completely clean them.
I have spent years learning and removing Viruses and Spyware. Yes, every computer that I have worked on has one type or another infection on it. Call me and I can show and clean your computer so you will not have the slowness and errors that are common with viruses and spyware. Two free programs you should consider are Avira and BitDefender. These are always on the top of the list and the paid versions offer more features.