My company installs mcafee on all our machines. Late one night, a few years ago, while on a business trip in a hotel somewhere, I opened an ***** attachment I should have known better than to open. I realized my mistake almost immediately, but the damn virus or worm or trojan horse or whatever immediately (less than 5 seconds) disabled mcafee and prevented me from activating the "start" button, so I wasn't able to shut down the machine in the normal manner. After another 5-10 seconds, when I noticed hard drive activity, i pulled the plug and battery and shut the thing down that way.
I wasn't able to boot normally, so after a lot of effort in safe mode, i was able to access the internet. It was too late for tech support, and Mcafee's website was useless. Finally I went to symantic and downloaded norton. Once I ran it, even before it installed itself fully, it did a system check and a scan and cleaned out the offending worm, and I was back up and running in under 30 minutes.
The pricks in my IT department later had me de-install norton and they re-installed mcaffee, but I switched to norton for my home machines.