SEATTLE, Jan 28 (Reuters) - A variant of the MyDoom worm has emerged as the most devastating virus since last summer, and is likely to target Microsoft Corp.'s Web site, security experts said on Wednesday. Since appearing earlier this week, the worm, also dubbed Novarg or Shimgapi, has infected computers across the globe by enticing users to open a file attachment that releases a program that potentially allows other attackers to gain unauthorized access. The financial damage from the virus-like program -- from network slowdown to lost productivity -- is already being measured in the billions of dollars, according...