1. An antivirus component.
2. An antispyware component
3. A 2-way firewall
You can get these separately, if you want to pick and choose the best of each. However, an integrated internet security suite often loads faster and is less likely to noticeably slow down your computer.
Several internet providers ISPs have free internet security suites available as part of their service. However, they are not automatically installed on your computer. Go to your ISP’s website and follow the instructions for downloading and installing their anti-spyware program. Note, however, that internet security suites provided free by ISPs are usually not the pick of the litter, although they are better than nothing.
Real-time vs on-demand. Some anti-malware software run full-time on your computer, in the background. These are called real-time applications. Other anti-malware programs only scan when you tell them to do so. These are called on-demand applications.
Real-time applications attempt to immediately identify malware when it enters your computer and to remove it before it can install itself or do any damage. Kaspersky Internet Security has been consistently better in testing than any freeware programs and almost all other propietary (paid) programs. Bit Defender is another security suite that has been getting top results.
On-demand applications are useful for finding malware that has already infected your computer - sort of like taking antibiotics after you have an illness.
Real-time anti-malware applications offer the best protection, but are never 100% effective (it's a war out there) and on-demand applications can be used to find and remove malware that your real-time applications may have missed. Because even the best internet security suites are not 100% effective, on-demand antimalware applications can help. Malwarebytes Free (www.malwarebytes.org) is a free on-demand anti-malware application that seem to work very well at catching what falls through the cracks.
Keep your definitions updated
Anti-malware software publishers spend a lot of time researching malware and keeping up on the latest variants. They publish updated "definitions" frequently. If you don't keep your malware definitions updated, the programs cannot protect you against the newest variants. Most anti-malware programs come with a 1 year subscription to their updating service. Where things fall apart is when users get too cheap to renew the subscription, or don't bother to register their program. Without registering and having a current subscription, anti-malware programs cannot download the newest definitions, and cease to protect your computer.
Although backing up your data is an important security step, your CD or other media containing you backed-up data may have malware files on it and restoring your data after a particularly nasty virus trashes your computer, just puts the virus back on your computer. So pay a few bucks and make sure your anti-malware subscription is current.
When it comes time to renew your anti-virus program's subscription, if you shop carefully. Some clients have told me that they wait until a rebate is available and then buy the newest version of the software - complete with a 1-year subscription - for less than the cost of the annual subscription fee, after rebates. Keep your eyes and ears open.