Tom Spring, PC WorldMon Jul 18, 4:00 AM ET Tip of the MonthIs your PC running malicious code that may have turned it into a spam-spewing zombie? Microsoft's Malicious Software Removal Tool scans, identifies, and extracts threats. Symantec also offers a free online virus detection service and a software program that identifies existing problems on your PC and determines how safe it is from potential online threats. Symantec also offers dozens of virus-removal tools. In a novel if potentially controversial effort to fight spam, a firm called Blue Security this week begins distributing the beta of a free program that,...