Beginning March 2007, daylight time is in effect a total of four weeks longer than previously. Computers running Windows XP or older operating systems need to be updated so your computer time is correct. Vista and Windows 2008 contain this update but later time zone updates are available for Vista and Windows 2008.
If the time zones are not updated for the 2007 DST changes, appointments created in US time zones and scheduled during the last 3 weeks of March and early April will be off an hour.
The most recent update at the time of publication is December 2008 cumulative time zone update for Microsoft Windows operating systems. This update for Windows 2008, Windows 2003, Vista, and Windows XP and contains changes for time zones worldwide.
Windows update will install a patch that reflects the new daylight times, on Windows XP and 2003 systems. Users of older OS's will need to make the changes themselves. (Vista and Windows 2008 contain the 2007 changes to the US DST but the latest update should be installed so worldwide zones are current.)
Use the Time Zone update tool for Outlook to fix your appointments. This tool was first introduced in Outlook 2007 RTM then released as a standalone tool for all versions of Outlook. Office 2007 SP2 disabled the built-in tool. (This tool is also good for changing appointments if you move to a new time zone.) There is a similar tool administrator can run on Exchange server.
Verify the update is installed in Windows XP
There are (at least) three ways to verify the 2007 DST updates are installed in Windows XP or Windows 2003.
1. Check the time zones list in the registry. If you have a Dynamic DST key under any of the US or Canadian zones (as well as for a couple of other countries), it was installed. The time zones are located at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\.
2. Check for old and new versions of Mexican time zones in Window's Time and Date applet. Double click on the clock in the tray area, select the Time Zones tab and look for the Mexican time zones.
3. Look in Add/Remove Programs. Add a check the option on top to show all updates then look for KB 928388, 929120, and/or 955839.
Outlook Time Zone Update Tool
Microsoft released a tool that will update appointments when you permanently change time zones. It works with all versions of Outlook. Outlook 2007 includes an earlier version of this tool (and SP2 removes it) but Microsoft recommends using downloading the new tool for best results.
Exchange Time Zone Update Tool
Administrators can update appointments for all users at once using the Exchange Calendar Update tool. This is similar to the Outlook tool but is used against the server.
More information and download:
How to address daylight saving time by using the Exchange Calendar Update Tool
December 2008 cumulative time zone update for Microsoft Windows operating systems - update for Windows 2008, Windows 2003, Vista, and Windows XP
Daylight Saving Time Help and Support