I was using Norton Antivirus from the time I started using a computer and once I got to know about how it uses up system resources I checked the other best AVs available at the time, and if I'm correct Kaspersky and Bitdefender were on top at the time. I tried out both, but bit defender gave me so much trouble that I had to format my computer a several times and then I realized Bit Defender was the cause of it. It had a huge incompatibility with the system I had, or with the windows version I had at the time.
Then I went on with Kaspersky till the begining of last year. Had a huge issue with it, whenever it was updating it used up my processor 100% ( I had another laptop at the time, not the one mentioned in my signature, it had similar specs but the processor was a centrino2 core2duo)
So the system started to freeze everytime it was updating, and then it was not only when KIS was updating, it started happening very frequenty which got terribly annoying and no matter how many times I contacted the KIS support, they couldnt identify the problem. Then Avast was recommended by a friend and I was reluctant to try it out at first, but I installed the free version and was fully satisfied, it speeded up my computer greatly! I finally decided to purchase the Internet Security later on, and I have to say, I'm thoroughly satisfied with this product and would fully recommend it to ANYONE!
Also there was a recent experience I'd like to share, I had a malware attack and Avast kept detecting it as a network threat but couldnt pin point to the cause of it. Then with every scan it said I had threats in my memory modules - programs that was running, Skype, chrome and several others. I wanted to see if Kaspersky would detect it so I removed Avast and installed the trial version of KIS 2011, updated it fully and scanned the computer, it didnt detect anything! I was really happy that Avast atleast DETECTED there was a suspicious activity! Then the forum moderators/ senior members of Avast! informed me about Malwarebyte's antimalware (MBAM) and it got rid of the problem for me and everythings back to normal!

Avast IS + MBAM pro = the best protection you can get IMO

I also have SAS pro but disabled the real time protection as 3 resident programs would cause a conflict ( I had them installed for a couple of days but nothing went wrong, still better stick with 2..) PC performance also increased significantly after removing the threats with MBAM.