Obstinate. That's how Microsoft has labeled Conficker, which, despite being three years old and targeted for eradication, continues to survive--and even thrive--in corporate networks.

As recently as the fourth quarter of 2011, Conficker variants launched 59 million attacks against 1.7 million unique PCs, according to the latest installment of the Microsoft Security Intelligence Report, which reviewed attack trends for the second half of 2011. Whereas most malware disproportionately affects consumers, the report found that Conficker is "more prevalent on domain-joined computers," meaning business machines.

Here are eight reasons why killing Conficker remains so tough: