at the command-line, type the following sequence of commands
c:\w32tm /unregister
c:\w32tm /register
c:\net start "windows time" (quotes are required)
c:\w32tm /resync /rediscover
c:\w32tm /update
this will unregister the time service, re-register it with default values in
the registry, then start the windows time service, rediscover network
parameters, resync the clock using external source, then send message to
time service to update itself from the new values in the registry.
after that, you should verify that an internet NTP server is specified on
internet time tab of date/time control panel applet. the service can also
be modified to update more frequently than every 7 days (default), and much
more, let me know if you're interested in how to do that.
cheers