Suspicious AntiVirus Alert Popped Up on Web Page

I am suspicious about messages that popped up when I went to a web page I'd never been to before. First a message popped up stating "Microsoft Antivirus has found critical process activity on your PC. You need to clean your computer to prevent system breakage."  And then right afterwards another message appeared -- I might have pressed okay --  in a rectangular box with a thick red stripe across the top that read "Potential Threat Details," and then below "Microsoft Security Essentials detected potential threats that might compromise your privacy or damage your computer. You need to clean your computer immediately to prevent the system crash."  But I noticed that "might" was spelled "mihgt." Then below that it listed the threats as:

Trojan PSW.Win32Launch

HackTool:Win32/Welevate.A

Adward.Win32Fraud

It said each of the three was "critical" and "active" and recommended I "remove" them.

Then below all this was a box I could check that said "clean computer."

Now aside from the misspelling of "might," I do not have Microsoft Security Essentials; I have Windows Defender, so I'm pretty sure this is a fake alert.

And I didn't press "clean computer." But I'm wondering what would have happened if I had; would I have gotten a virus, or a sales pitch to buy more antivirus software? But mostly I want to know what really will happen when my antivirus program -- which, again, is Windows Defender for Windows 8 -- does detect a real virus. It hasn't happened yet so I don't know, and I also would like to be able to distinguish the real alerts from the fake ones, especially if the fake ones do not have misspelled words as a telling sign.

Thanks