If you have your own Windows PC, you must make sure that you have antivirus software installed.
Download the installer for the antivirus program you are going to use and save it in a suitable location (such as Downloads or Documents). Download links for some popular antivirus programs:
Choosing the right version
Make sure you pick the right version to download. Avast! and AVG offer basic free antivirus but also trial versions for more advanced software.
You might also be asked to choose a particular version depending on the type of Windows software you have - if you're not sure which version to pick, see: how do I tell which version of Windows I have?
It's always important to remove any existing antivirus software before installing a new one. New computers often come with a trial version of antivirus software. If you decide not to buy it when the trial expires, you should uninstall it and replace it with one of the free programs.
You can uninstall an antivirus program using add/remove programs in the Control panel or, if it is available, with a specialised removal tool provided by the manufacturer.
When complete, go back to the installer you downloaded earlier and double-click on the file to install it.
When the installation has completed, the software will update itself with the latest information about the viruses it needs to be looking for. This can take some time (up to twenty or thirty minutes) but when it's finished, it should confirm that you are protected.
Most antivirus software gives you the opportunity to run a scan of your computer to check for potential problems. This can take a long time to complete (several hours for a full scan) but it's a good idea to do this soon after you install a new program.