SAN FRANCISCO (CBS.MW) -- Sun Microsystems co-founder Bill Joy, a driving force behind the company's key technologies, said Tuesday he's leaving after 21 years. "I have decided the time is now right for me to move on to different challenges," Joy said in a press release. He was not immediately available for further comment. Sun shares fell 7 cents to $4.17 in early trading. The stock is up from a trough of $2.34 a share in October 2002, but is down from its 52-week peak of $5.64, set in June. Sun took its usual position among the Nasdaq Composite...