Windows XP, Vista and Windows 7 …
Have a registry value you can check to see if a Time Zone update has been applied.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones registry key.
For Windows 7 and Vista, this is the TzVersion key.
Windows XP, it’s the KBLast key.
Windows 2000, is a mystery to me.
For the September 2010 Time Zone Update, the KBLast key is set to 131728128.
In the screenshot above, the KBLast key is set to 955839 . Since that update (December 2008), it looks like Microsoft have changed the versioning scheme.
Update 1st October:
Added Windows Vista details.