The MSBlast worm, also commonly called the Blaster worm, started spreading last August and is believed to have spread to hundreds of thousands of systems. While most corporations have cleaned up the worm, Microsoft has found that a large number of home users are still unknowingly infected, the software giant said in a statement.

"For many users in this situation, there is little indication that they are infected other than possible performance degradation," Microsoft said in a statement. "And those infected are still actively transmitting the worm, causing Internet congestion in the process."

Worms--programs that spread between vulnerable computers over a network--have been the bane of Microsoft since Code Red and Nimda hit computers running the software giant's operating system in summer 2001. Nimda, which hit computers a week after the Sept. 11, 2001, terrorist attacks, put the company on the path toward making security its No. 1 priority. Four months later, Microsoft announced its Trustworthy Computing Initiative to do just that.

Robert Lemos at C|NET.

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