Terminal commands and operating system's viruses etc.

Question

I was wondering what are some good and cheap/free,  Anti-virus or malware detection program out there, that  I should be thinking about getting? I guess I
would consider myself a person that gets around on the Internet and and downloads and does where he pleases.
I have downloaded a few app's that should help me like Memory Freer, Clean My Mac and DaisyDisk. they all seem to be very helpful and all but I don't think there scanning for the hidden nasty's that crawl into you're computer and break, corrupt and slow it down.
I've also been learning Terminal and all the commands and action's that can help also when you get into a sticky and sometimes very irritable situation.
I haven't been Using it for that long and was kinda scared to. But I know it helps and I actually kind of enjoy it.
So also  If you guy's know Unix and some of the commands, what are some of the really important ones, Just incase I accidently open the wrong link and all of a sdudden there is a program zip file from the Internet unzipping and loading up then disappears on me, Like has happen a few times? up or something,also if you think you'r computer has been intruded, a program has been hacking think someone got into you're system file's or a bad link gave you something, What are the command's to see that kind of thing?
Also one last question. I originally bought my  3.06 mhz, 1TB HD 4GBRAM  27" Late 2010 Imac from an older guy that had bought it with Snow Leopard with and then upgraded it to lion, before he sold it to me.  My computer crashed a week or so ago and I did'nt have the lion CD so I went ahead and put Snow leopard back on it until I was able to get the  the extra  $20 dollars for the Mountain Lion upgrade.
When I did that I was able to fell the difference of the vibes of the GUI, and also was able to see the differences in a few thing's involving some of the features and lay out's.
For instance; The applications and documents folder is on the dock in in snow Leopard and you can scroll through the folder with out even having to opening it up.   and when I finally Accuired the 'Mountain Lion up grade app. After I upgraded it again it didn't even seem like the same OS anymore i seemed like it was a morphed version of the Leopard GUI and lay out. So is it Different everytime you load up the different Operating systems in different orders?
I forgot to mention that I actually had upgraded it from the previous owners Snow Leopard/ OSX Lion OS,  To The Mountain Lion.I was Using a upgrade that was not licensed to me.. and that's when the Computer completely crashed and had lost all key's and permissions to OSX in the mayhem that followed.
I guess my qustion is should I go back and And start from the begging and install snow leapord then Lion adnd then Montain lion? Will it make a difference in the feel and operation?` or ` during the recovery process or after is ther a way to set it up the way you like it? Like if you wanted it to feel more like SL or like Lion?
Thank you Very much for the info in advance.

Answer 1

You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful: The User Tip seeks to offer guidance on the main security threats and how to avoid them.
https://discussions.apple.com/docs/DOC-2435