So I already have F-Secure of some sort, it's called Elisa Tietoturvaplavelu which means Elisa (a company) Security service or something along those lines. I am not sure if I am infected or not as my PC seems running slower these couple last days. Once I tried to go to overclock.net but wrote overclocl.net and it went to this Gongratulations you won! web site which after a little research is a malicious site. On this site they say that it would only infect you if you click on it which I didn't do (the site: http://blog.mitechmate.com/congratulations-you-won-com-virus-removal/ )
I just might be paranoid and nothing is happened but I never can't be too sure!
AV-Comparatives (http://www.av-comparatives.org/) is probably what you're looking for. They do AV tests on a regular basis to see how effective they are. I'm currently using BitDefender based on their larger yearly results.
There are way too many reasons a web site might be reported as malicious. My best guess is that there's some sort of problem with an encryption 'certificate' (used for HTTPS). It could be due to malformed cookies, scripts or something along those lines too/instead. It might even be due to a web browser add-on/extension or even a DNS server which you have absolutely no control over (other than which one to use). And if you regularly visit risky sites like porn sites -- particularly with old versions of Flash/Java -- then you would indeed be at a significant risk of something bad getting on your system. And we still haven't got into how you connect since it could even be something as simple as your ISP messing with you.
That said, I still doubt you have an actual "virus" particularly if you're running any kind of updated virus scanner. The very fact that your PC is even working pretty much tells me that.
So any sort of slow down your Windows PC may be experiencing would more likely be due to fragmented files which now take longer to load now that their contents are "fragmented" all over your hard drive. It might also/instead be due to more "processes" running -- all of which is quite normal. So unless you're seeing weird messages or your PC is it's wigging out for some reason, I doubt you have anything to worry about from the world of viruses.
Now, if you're concerned about SECURITY then that's almost too big a topic to cover. Things like malware and spyware are more likely to be culprit's with regard to system performance. Just having an anti-virus product installed and updated may not be enough -- even on a Linux box! In fact, most AV/AM products themselves will rob your system of performance. So make sure you don't do stupid things like install programs from unknown publishers etc. Don't wait forever to get updated. Keep your firewall on and you may even want to tweak your browser to not allow Flash/Java too. Clear your caches is still another safeguard. And I'm just getting warmed up. Point is, don't assume any anti-virus product is going to keep you safe -- it's not! There is no magic pill. Knowledge and maybe even good habits are all anyone has to rely on -- real or cyber.
If you just want something for piece of mind, install MBAM and run some scans. It's free, easy, effective and wont interfere with your existing AV suite since it's not active protection.
Trendmicro has a free online virus & malware scanner called 'Housecall' that has helped me in the past.
As far as antivirus goes ... my favorite free one is Avast (because many of my clients are charity cases lol ) it does the job and I personally use Avast Internet Security Suite and am very happy with it. I personally can't stand Norton or McAfee ... but this is just my opinion.