The tool added detection and deletion skills for two more pieces of malware, the worms dubbed "Cissi" and "Fizzer." Both have been rated by Microsoft as a "moderate" threat. Interestingly, Fizzer is more than three years old.
Microsoft uses a variety of criteria to select the threats it sniffs out, including data acquired from its Windows Live OneCare security service and Windows Live Safety Center scans, customer feedback, crash alerts forwarded to the Redmond, Wash. developer, and problems raised with its support desk.
In a recent analysis of data from the first 15 months of the tool's operation, Microsoft said that about 1 in 311 PCs that run the utility are infected with at least one piece of malware, but that it has drastically reduced the prevalence of worms and Trojan horses on Windows-powered computers.