It seems those virus scanner or detector slow down the PC system sometimes  

It is true that those virus programs do slow the performance of your computer especially when it is doing the scanning part ! but yeah perhaps you could try Avira Anti-virus ! it is the one of the anti virus programs I have used that does not use up too much resources of the computer! perhaps this one will work better with your computer. Secondly it is possible that your computer does not have enough ram on it :)

Have you tried looking into ESET Smart Security? It has everything you need; Anti-Virus and Sypware, Malware, Anti-Spam, and Personal Firewall that is less annoying than Windows Defender/Firewall.

The firewall feature for ESET Smart Security is very powerful, you get a prompt for any and every incoming and outgoing connection to your computer, you can even add exceptions or rules to allow or disallow even your web browser to even connect to the net.

All in all it's a very power Anti-Virus suite; and it uses less memory than windows explorer!

The reason for your computer slowing down or seeming to run slow is really because some virus scanners are constantly running scans throughout your system, and every time your surf the web.

virus protection systems that I highly DO NOT recommend are,
Norton AntiVirus
McCafee
and any others sold at Best Buy or other places.

Norton was reported to slow down a computer system up to 50%
McCafee - was something like 25%

But if you are looking for a Free AntiVirus software that does work really well, take a look at Avast Antivirus.

I run Avast on all my family's computers and on my own, and I highly recommend this software to anyone looking to get a quality antivirus software that won't turn your pc into a slug. lol

(I used to run AVG before Avast)

Yes somehow the virus scanners affects the speed of browsing.Because it is screening every pages your clicking.Checking for harmful spywares or trojans.They are still program eating up some space in your RAM, but we need them ,so we have to live with them and just clean up our trash cans regularly to somehow take care of the speed of the system.

Yes man there is virus scanners which can slow down the computer performance. Specially I have found that Norton is a huge resource hog. It does very little and uses too much and that is not something I am comfortable with. I mean I am not wasting my resources just to keep my computer scanning. It should be light on resources and yet very efficient. And that is why I am using NOD 32.

i have tried avira and avg.. looks like avg is much slower. but you can adjust the scanning progress. in slow scan, it will take longer but doesnt affect your computer much. there is the normal scan and the fast scan. the fast scan slows down the computer. some say that when you use fast scan, it doesnt scan the computer very well.