A thing to remember is to never click on those things that pretend they are scanning your computer when you didn't initiate the process yourself. They pretend they are scanning your computer and then say something like "Click here to remove... " when you yourself didn't initiate any scan. DO NOT CLICK on those adds. They are what's known as scare-ware, where they trick you into clicking on a link that winds up giving you the virus/ malware.
Fully scan your hard disk drive for anti-virus software and make sure everything's working properly.
Clean up your computer too if you can ie. run disk defragmenter and see what's installed on your hard disk drive.
Another thing you could do is see what programs are running in the background, that slows a computer down too.<br / />