Computers these days come pre-installed with a plethora of Antivirus software. Usually most are just trials, they run for about 3-15months before you have to renew for a yearly subscription. The problem I have come to find out is these Antivirus software use a-lot more system resources, causing your computer to perform slower than it was intended. Of course the computer will continue to function but it could perform its daily tasks a tad faster if other options were tried.
Microsoft now offers a free alternative, Microsoft Security Essentials(MSE). If you have windows 98,xp,vista,7, then you can acquire a free copy from their website. Another good spyware/trojan/virus protection that does a good job is called Malwarebytes(MB). I have used MB in multiple occasions to clean client computers that were infected and it never disappoints. The free edition of MB runs on demand(user request) while the paid version is real-time(always protecting).
Notice the choices above? Both AV systems are light weight, meaning they are not system resource hogs. Technically you could install both AVs without causing lag on your system, infact I would recommend running Malwarebytes Free/Paid Edition & Microsoft Security Essentials simultaneously. If you are looking for just a single AV to protect your computer I suggest you check out Avira, there is a free version available too. Avira also has a rescue cd that boots the AV in livecd(linux) mode, it allows you to do a deep scan while windows is not booted for maximum effect.
Learn more about the AVs out there, check out AV-Comparatives.