Damaging Terminal sudo session?

Question

I’m new to Mac (less than a year), and to administering my own computer. Trying to get things right and agreeable to me, for security and utility, is taking up much of my available time—as much as, or often more time than I spend USING my Mac.
Recently, I tried to use a sudo command in Terminal for the first time, following advice published by Macworld for forcing updates to OS X’s new XProtect malware definitions, and I wonder whether I’ve done any damage by what I did, from knowing next to nothing about command-line protocols, how to exit Terminal properly, and so on. I have a spring 2011 MacBook Pro, running OS X 10.6.8 (it may have been running a prior update version when I got myself into that maybe-troublesome Terminal session some weeks ago).
The command I ran, per the Macworld article, was “sudo /usr/libexec/XProtectUpdater,” which brought up this typical warning: “Improper use of the sudo command could lead to data loss or the deletion of important system files. Please double-check your typing when using sudo. Type ‘man sudo’ for more information. [Which I did, haplessly: I didn’t then know how to get further down that file than what was appearing in the window!] To proceed, enter your password, or type Ctrl-C to abort.” After my visit to man sudo and maybe then having to paste in the sudo command again, to run it I entered my admin password. As no characters were appearing in the Terminal window for the password, I thought nothing was happening; but I pressed the Enter key (or the appropriate button in the password pop-up, whichever was the case) anyway.
When (eventually) I tried to close the Terminal window (without knowing at the time about using the exit command), I got this pop-up message: “Do you want to close this window? Closing this window will terminate the running processes: …” followed by a short list that included, I think, login, sudo, man, maybe bash. I chose to close the window anyway, and immediately worried that I may have done something I shouldn’t have.
After running that updater command again on one or two other occasions since (though those times exiting Terminal through the exit-command route), I’ve since read on macworld.com that using that sudo command to force malware-definitions updates “can cause users to lose their login.keychain file.” (Thanks for the initial generally-broadcast advice to use that sudo command, Macworld.) I don’t yet actively use the Keychain for storing passwords, so I thought, no problem.
When still at the computer right after that first, half-witted occasion, however, and switching between my standard account and the admin account, the desktop filled only about  80% of the display, from the upper-left corner; the remainder of the display below and to the right of the desktop was just plain blue screen. I’d never experienced THAT before. That got remedied by a restart (though I think I remember it happening that night when logging into both the admin and the standard-user accounts), but it has recurred often, seemingly now only when I log into the admin account. Each time it does happen, a restart fixes it. But I think it’s a result of my fiddling unknowingly with that sudo command, could that be?
Next, a little more knowledge maybe yields some more trouble: I’ve tried, as admin user, to look at the Mac’s /etc/sudoers file to see whether it contains my admin, or any other recognizable user, but I get this response: “Permission denied” and ditto for trying to look at (or run) /private/etc/sudoers, private/etc//sudoers (which I saw referred to in these forums, I think, and wondered that it might NOT be a typo with those two slash marks together), cat /etc/sudoers. But I’ve read that admin users have sudoer permissions, no?
Advice? (about damage from my Terminal sudo and botched-exit misadventure, the 80% desktop in the display, my inability to access the sudoers files)

Answer 1

The problem with the display seems to have occurred when logging out from one account (either my standard-user acct. or my admin acct.) and (though maybe just sometimes, not always) into another account. I hadn’t checked the screen resolution until following your suggestion, and found it at 1440 x 1052 (stretched) when in that ~80%-of-normal condition. This display’s nominal resolution is 1680 x 1050. When 1440 x 1052 (stretched) resolution is chosen normally, however, the desktop or login screens appear symmetrically onscreen, centered and using the full extent of the display, unlike how they appear when this problem occurs “on its own.” Since resetting the resolution (when the display was in that problematic state) to 1680 x 1050, last week, the problem hasn’t recurred, through several logouts and logins between these two accounts, in a couple of different sessions. Thanks for the helpful advice.
The F9 key function doesn’t seem to have had anything to do with this display problem. Your wondering about that, though, and the advice in your last two paragraphs, are worth my attention generally. I appreciate the links to Apple’s instructions on resetting PRAM, NVRAM, and the SMC.
So, the display problem that may or may not have come up from my half-baked Terminal session seems solved, but my worry or wondering remains over what may have occurred from my sudo fiddling to get malware definitions updated. And why my admin account can’t access the sudoers file, and whether that’s another problem, I dunno, and I don’t have a lot of time to be getting into finding out, yet. I wonder whether I should just reinstall the OS and start fresh.