Where did the AV come from?

I have another AV software running (with realtime protection), and when I installed Comodo Internet Security, I installed only the firewall and Defense+ with maximum security. Since the last few days, every time I shut down, I would get a BSOD, with nothing particular written so as to understand the problem, though it did have the following stuff written at the bottom - 0x0000007E (0xC0000005, 0x8066575B, 0xF78C2BD0, 0xF78C28CC). I would have to manually press the power button till the CPU shut down. Today I was looking at the options in CIS, and I found one in the main "Miscellaneous" tab, called "Diagnostics". I clicked, and it did some kind of checking and said there were problems and would I like to fix them, so I clicked yes and it fixed whatever problems there were. I later rebooted, and surprisingly, I didn't get a BSOD (which I also got during reboots). Later when I went to Defense+ to unblock a folder I usually block, I found that the whole list was empty, that the Diagnostics tool had wiped out all settings and everything. So I had to open the forum page again and set the options for Firewall and Defense+. It was at this time that I noticed that in the system tray of Windows, when I right-clicked the CIS icon, at the top where there were earlier only two options of Firewall Security Level and Defense+ Security Level, there was now another one, right at the top, which said Antivirus Security Level, and it was set to Stateful. I immediately disabled it, and found a guide here about disabling the AV, by going to the main Antivirus tab and setting the Scanner Settings to Disabled.

What I'd like to ask is, I never installed the AV during setup, and I don't remember seeing anything related to the AV earlier, so how come it has popped up now? Has the AV engine or whatever got installed when I ran the Diagnostics? And how do I get rid of the whole AV thing? Is setting the slider to Disabled in the Scanner Settings enough?

Thanks.