Viruses Aimed at Microsoft Rise Sharply-Symantec  SAN FRANCISCO (Reuters) - The number of new viruses and  worms aimed at Microsoft Corp.'s &lt;A HREF="http://www.reuters.co.uk/financeQuoteLookup.jhtml?ticker=MSFT.O qtype=sym infotype=info qcat=news"&gt;MSFT.O&lt;/A&gt; ubiquitous Windows  operating system rose 400 percent between January and June from  the same year-earlier period, leading computer security company  Symantec said on Sunday.