Step 3. Use antivirus software

The Danger: Computer viruses can cripple your computer and destroy your data.

The cause: There are an incredible variety of computer viruses on the Internet, with many different ways of infecting your system. The stereotypical vector for viruses is the email attachment, and this is still the most common source of infection for unwary users. Opening up a seemingly innocent attachment from a friend can have disastrous consequences. There are few computer users who have not experienced the effects of a computer virus at some point, and malicious coders keep churning them out.

The cure: Install and use a professional antivirus program

Using an antivirus software package from a reputable manufacturer like Symantec or MacAfee provides an effective defence against viruses. You should scan your system for viruses once a week at least, and use the software to examine any email attachments you are unsure about. Many packages, like Norton Antivirus, come with auto-protection features which will scan any files entering or leaving your system for viruses.

While this is fairly self explanatory, a couple of extra tips: if you're going to buy and install anti-virus software, do it now, before you get infected with a virus, rather than waiting until your system starts to act up. The reason for this is that many viruses have components that can disable or subvert popular antivirus programs like Norton's and MacAfee's software. So if your system is infected before you install the antivirus software, it may not be able to help you.

Secondly, make sure you keep the program updated. Antivirus software manufacturers are constantly creating new sets of virus definitions to keep up with new threats. Without updated definitions, the software will not stop newer viruses from infecting your PC. Most reputable antivirus programs will update themselves automatically when you are connected to the Internet, but it doesn't hurt to make sure you have the latest update before you scan for viruses.

Step 4. Check your PC for spyware and adware

The Danger: Spyware and Adware programs can quickly infest your PC, compromising privacy and performance.

The cause: Spyware and adware are generic names for a variety of programs designed to collect data and/or advertise products. Sound innocent? The catch is that these programs are often installed on your PC from websites or as part of 'free' software like Kazaa, and work from inside your computer, gathering information about your surfing habits for marketing purposes. Worse, this is only the tip of the iceberg.

Programs in this category may call up extra pop-up advertising while you are surfing, or even redirect your browser to websites of their own choosing. While makers of this type of software need to obtain your consent to install their programs, they are often presented in misleading ways, or hidden within the license agreements of other software.

There is a second category of programs involved as well, browser hijackers. These malicious programs can subvert your web browser's home page and links and generally cannot be removed without great difficulty. At their worst, these programs can make using your computer a trial. System and Internet performance can be slowed due to the extra data being sent from your computer, and floods of (often obscene) advertisements may dog your Internet surfing experience.

For more details, see PCSTATS guides on spyware, adware and browser hijackers.