To change the details stored for a time zone (eg governments decide to move the daylight savings start time) you can download a patch from Microsoft but if you are using an older version of Windows this may not be available.

The alternative way is to use TZEdit, a Microsoft time zone update utility. This is freely available from Microsoft and allows you to change the details for each default time zone for every version of Windows.

The thrid method for adjusting time zones is to modify the registry directly. The applicable registry entries are:

Default time zone: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\Time Zone name

Currently loaded time zone: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation