How do I stop this sytem error pop-up thing?

I keep getting this thing that pops-up ALOT and its annoying this is exactly what it says:

System error

Your system is infected by a dangrouse virus

Note:Strongly recomended to install our antispyware program to clean your system and avoid total crash of your computer!

Click OK to download the antispyware. (Recomended)

I understand its just trying to get a run for your money I just wanna know!

1.How did this happen

2.How do I stop it

Thats all and if there is any way other then downloading stuff to get rid of this pest let me know give me all the ways!

Thanx!!!

How do I stop this sytem error pop-up thing?free anti virus

Phony pop-up alert boxes, background changes or ''balloon'' pop-ups from the tray (by the clock) about a trojan, virus or spyware infection and the attempt to sell you some useless software to ''fix'' the problem usually indicates that your computer has a Smitfraud infection. DO NOT DOWNLOAD OR BUY THE PRODUCT THEY TRY TO SELL YOU! IT IS SCAM. It is really extortion-ware, first they infect your machine, then they want $$$ to remove it.

Sounds like your computer may have a variant of this ubiquitous Smitfraud infection.

Almost all ''general purpose'',''all-in one'' spyware and virus removal programs such as AVG Anti-virus, Norton, McAfee or Spybot S %26 D cannot remove it, as you may have already found out.

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