I myself am personally tired of typing out the horror story that I had to go through because McFubar came with the PC. It wasn't pretty.
Suffice it to say, even 2 months after removing it (so I thought), the deep seated remnants of it were still causing problems with legitimate safety tools (Malicious Software Removal Tool, MSE, Malwarebytes, etc). After a 2 day project of tracking down the offending files, I finally eliminated McFail from the system once and for all.
But to add a story....let's see, there was the time McFlimsy hosed all those XP machines by identifying a legitimate Operating file as spyware/malware....and then, not satisfied with that, decided to hose Vista machines a couple months later....
Or the times I brought my FD home from school & MSE would immediately identify a threat on it, while repeated scans with McFlop would show it as clean.
I don't want to say anything bad when it comes to AV's, since we all have our own preferences. But, you won't find many positive posts about McCrappy in this forum.
Just remember, having a good AV doesn't replace common sense & exercising caution.