Kanwaljitji , if you are running several at the same time, all scanning, your pc will slow down to a crawl, Malware bytes free version does not have an active scanner, you just run it when you feel you have problems, same with combofix, also, dont forget if you are running lavasoft ad-aware together with an antivirus program then you run the risk of neither of them working at all, You cannot run two different antivirus programs on the same pc at the same time, they will conflict, as ad-aware has antivirus built in.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/information-technology/38507-spyware-problems.html

Sophos is the market leader insofar as antivirus is concerned, together with a few weekly scans by malware bytes, I cannot recall the last time I had problems,
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=38507