For what it's worth here is a quote from another page on how to change it on XP/2k. I've read this no longer works with Vista and can confirm it does not work on Win 7.
Control.exe TIMEDATE.CPL,,/Z (GMT-08:00) Pacific Time (US & Canada); Tijuana
alternatively:
RUNDLL32.EXE SHELL32.DLL,Control_RunDLL TIMEDATE.CPL,,/Z (GMT-08:00) Pacific
Time (US & Canada); Tijuana
where '(GMT-08:00) Pacific Time (US & Canada); Tijuana' is the timezone to be
set.
Input value behind /Z is from the Display value under the registry key
'HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones'
You can use the Std value in those registry keys as well, this is equivalent
with "(GMT-08:00) Pacific Time (US & Canada); Tijuana":
Control.exe TIMEDATE.CPL,,/Z Pacific Standard Time
