No antivirus protection can claim to be infallible – and indeed there are products out there which definitely have weak spots, as you’ll see from the results security suite testing labs such as AV Test regularly publish. So even though you’ve got protection up and running, there’s a chance that your computer could still get infected by a virus.
So what should you do if your machine suddenly starts behaving oddly, assaulting you with multiple pop-ups or running sluggishly (or all of the above), and you suspect you’re a victim of malware that your security product hasn’t detected? For starters, you can make sure your suite’s virus definitions are totally up to date, and also ensure you’ve run a full scan (which takes in everything on the PC – you’ll have to wait a while for it to complete).
If there’s still nothing detected, then it’s time to look to a third-party solution. Try downloading and using Norton Power Eraser, which is a free download. As Symantec says, it “eliminates deeply embedded and difficult to remove crimeware that traditional virus scanning doesn't always detect.”
When tech support agents for some security vendors run into a problem their own product can't handle, they sometimes turn to Malwarebytes Anti-Malware for help with clean-up (there's a free version, and a Pro one). If the real problem is that the malware fights back against removal, try a free Rescue CD that boots into a non-Windows operating system and thereby prevents the malware from running. For example, you can grab AVG’s Rescue CD here.
One of these simple and easy to use tools should hopefully help to shed light on your situation. Another option, if you’re using one of the less reliable security vendors according to AV Test’s regular evaluations, is to switch to a different product – see our guide to buying security software, and our piece on the best security products of 2013 for recommendations.