What do you do when PC Security Software has taken over your computer?

I have seen several posts regarding PC Security Software and all of these suggestions on how to remove it, BUT what do you do when it has already infected your computer to the point where all of the icons are gone, it blocks access to control panel, and it tells you that you can’t do anything because spy ware has infiltrated the computer? This is on my father’s computer, just trying to help him out! If any of you can help, that would be great!

Do Macs Really Need Anti-Virus Software? – Associated Content

The Apple Mac has long been safe from the world of computer viruses, but the once invincible Mac may now be going the way of Windows based PC’s. Recent developments have shown that Apple Macs around the world might be vulnerable to attacks. The Mac …
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My computer has been hijacked by a fake security scam and it overrides mcafee security how do i get it off ?

system tool security has taken over my computer, it runs a scan automatically and wants to get your credit card number. it has somehow bypassed mcafee securitly i can’t open mcafee at all. don’t know where it came from can anyone help me or do i have to take my tower to a geek squad place thanks…

How to get this fake security spyware crap off my computer..?

My friends computer has some kind of spyware crap on it and she cant get it off. It has taken off all her icons on her desktop and it wont let her go to anything but the internet. I have had something similar on my computer but I tried using what I used to get it off but it didnt work. Its like a fake security thing. It says she has all kinds of viruses but she really doesnt. Does anyone know a way to get it off! Help please!!