Is your system running slow, or is there abnormal activity within your computer; such as your webcam turning on randomly, or your hard disk whirring when you're not using it? If you have experienced any of this, then you probably have a virus! Now, don't be alarmed, it's probably happened to everyone, getting infected is pretty inevitable if you're not careful online. The internet is very vast and many hackers want to infect you to use for their malicious purposes. So it's your responsibility to keep your system clean, or your personal information might be taken be abused. Luckily for you, you're reading this article; the first step to keeping your computer clean.
When that's complete, update it; it should update automatically though.(When it updates, it updates the definitions; the definition is simply viruses recently found that are added to the database to look for when it's scanning) Now, once the installation and update is finished, unplug your machine! If you have a Trojan, signals can't be sent to your computer to control you if you're not connected to the internet, rendering viruses temporarily useless. Now click on "Scan your computer..." and select all the drives you mainly use. If you're not sure, just select all of them.
Now you have to simply press full scan(A full scan scans all the items in your system and compares it with its database to see if the file is infected). It will take some time, so you have to be patient. When it eventually done click "Show Results, and select all the viruses unless there's a false positive for a program you trust. But I suggest you don't do this because they can be imitated. Then after it's done removing the infections restart your computer. When your logged in, plug-in your internet.
Now, the next part is simple ways to keep your system clean. The first thing you should do now is update software that you may have. Such as Adobe Reader, Browsers, etc. And don't forget to and sure your windows is patched up to the most recent version! Lots of exploits are found and left in old versions.
Next, run quick scans weekly, both with SuperAntispyware & Microsoft Security Essentials. You can schedule it so it it does it automatically. Just make sure they aren't at the same time! This is so that if a virus comes back, its killed.
Then, install a firewall like Zone Alarm Free Firewall, and install it and configure it. This makes it so that the connections that go in and out of your network are secure and encrypted. It also blocks suspicious ports from opening like from a Trojan.
After, you need to do is get a better, more secure browser like Google Chrome or FireFox. If you choose to get Firefox, there are many plug-ins that help keep your computer secure. Such as noScript, which let's no scripts run on the page, but you can disable it for sites your trust. There is also AdBlock for those pesky little ads that always will bother you. There are many more plugins like this, for chrome as well. But DON'T USE Internet Explorer, because hackers are always trying to exploit it because most users it.
Lastly, use a Sandboxie whenever possible, or when you don't trust things. A sandboxie keeps all the running applications inside of the sandbox and nothing goes into your system. A good explanation of this is on their website. http://www.sandboxie.com/
Protip: Use Common Sense 2012, works wonders. Stops most malware before they can even look at your computer.
Hope you managed to clean your system and learn ways to keep your system secure!