Microsoft Warns on Spyware's Impact on Update By ALLISON LINN    SEATTLE (AP) -- Though Microsoft Corp.'s new security update package is all about protecting systems from worms, viruses and spyware, it can't do much about what's already on computers - and that could pose a problem.    The company is warning users of the Windows XP operating system to check for spyware before downloading the free massive security update, called Service Pack 2...