Why does Norton Antivirus cause problems for macs?

I am using a 2009 Macbook pro and I love it. Recently, I noticed it was wasn't working nearly as fast as should be. Unfortunately, I downloaded the free version of Norton Antivirus for mac, and I figured that might have something to do with it.

My question: What about Norton causes problems for macs?

Also, I noticed allot of itunes files had become corrupted during this period. I suspect the anti-virus software may have something to do with this. Why is that?

I am not sure about your corrupted files, but Norton and I don't get along on Windows either. It's not just OSX and Macs. I find it takes way too many resources (even more on Windows) and makes the computer jerk. With some work I do I can't deal with it. I moved away from Norton AV back when Peter Norton sold the company and eventually went with a free solution called AntiVir.

On OSX at this point at least, you really don't need an AV unless you want to find Windows Virus's in files you might send to say a friend as not to infect them. I just keep ClamxAV installed and run it manually from time to time.

Symantec (in general) hasn't produced a decent piece of software for Windows or Mac in the better part of a decade.

AV products in general will slow a machine down, because they employ heuristics (a form of artificial intelligence) to try to discern virus-like behavior. Otherwise they would only ever scan for the viruses they know about via their definitions (and of course on the Mac, those definitions are pretty scant). My guess is those heuristics are really aggressive as the performance complaints as well as the general instability caused, are rampant.