The United States kept its lead in producing spam last month, as the <A HREF="http://news.zdnet.com/2100-1009_22-5453138.html?tag=zdnn.alert">presidential elections proved a popular topic</A> for junk mailers. In October, 42.5 percent of all spam originated in the United States, according to a report released Monday by Commtouch Spam Detection Center, which analyzed hundreds of millions of unsolicited messages.