No other applications are freezing?
I concur with @DeanV‘s skepticism about the virus. A smart virus doesn’t want you to know it’s there, and is unlikely to deliberately freeze you. It could be some strange problem with Safari, but the safe money is on a configuration problem or hardware problem – maybe the latter if other applications are having problems. I never used Sophos, but I see no reason not to check for the virus, but it’s probably not the problem.
I don’t know if I totally agree with blanket statements that you don’t need a virus scanner. That depends on you. Normally you don’t, on any modern platform, but that depends on how you use the computer and how many other people use the computer and how they use it. Macs don’t have the security by obscurity “immunity” they used to have – nowadays they have that sweet combination of ubiquity and a largely stupid userbase that Windows traditionally had and still has, so they are being targeted by virus makers.