A computer is only as good as its user, and unfortunately, there are many, many, many less skilled computer users. To keep your computer secure, and therefore fast, there are some basic security practices you should employ.

Spyware/adware protection
Spyware and Adware are programs that are unknowingly installed on your PC. These programs track your activity on the Internet, in order to show you ads, pop-ups or obtain private information about you. They slow down your computer and can be a security risk. To prevent them, use a free antispyware program.
Recommended: MalwareBytes AntiMalware or Adaware from Lavasoft

virus protection
Viruses are more nefarious than spyware, because they are intended to directly harm your system and may be intended to steal identity information. Viruses replicate and spread unlike spyware. To fight viruses you need a reliable antivirus.
Recommended: Avira AntiVir and Avast! Home Edition are excellent free solutions

Hacker Protection
Hackers try to directly access your computer through security holes. Antispyware and antivirus programs do not protect against this type of threat, which is the most dangerous of all. If your Internet modem or wireless router does not have a built-in firewall, you should get your own free firewall.
Recommended: PCTools Firewall Plus or Comodo Internet Security Firewall

wireless security
If you use a wireless router in your home, it’s probably very easy for anyone within 200 feet of your home to access your signal. This can be dangerous if someone has bad intentions. Set a wireless password to access your wireless network. If you don’t know how to do this, ask for any manuals they have for your router or ask the company.

good passwords
Passwords are useless if they are not chosen well. A strong password must be longer than 7 characters and contain at least one number/symbol and a combination of upper and lower case letters. Also, you should not use the same password everywhere you have an account.

secure email
Be careful what you open. If you don’t recognize the sender or the subject, don’t open it, especially if the email contains an attachment. Common sense is the best medicine here.

By following these security practices, you should remain a safe computer user. Rest easy knowing you won’t be one of the many victims of online crime, nor will you have to use your hard-earned money to replace your computer every few years due to infections.