It is critical to make sure that your computer is running software that protects it against malicious software (or malware). Malicious software, which includes viruses, spyware, or other potentially unwanted programs, can try to install on your computer any time you connect to the Internet. They can also infect your computer when you install a program using a CD, DVD, or other removable media. Malicious software can also be programmed to run at unexpected times, not just when it is installed.
Different options for preventing these programs from infecting your computer exist, and depend on which version of Windows your computer is running. It is typically best practice to ensure that you only have one antivirus program installed at a time.