Also:
The first thing I would check is the time zone and date. Make sure your date is current and that the computer is set for the time zone you live in.
If that is all set correctly and you are still losing or gaining an hour, look for any program you have installed that might interact with your clock such as appointment reminders, calendars and so forth.
Windows XP and 2000 can synchronize a PC with atomic clocks connected to the Internet. Make sure that feature is turned on. Double-click on the time in the system tray, then click on the Internet Time tab. Make sure the "Automatically synchronize with an Internet time server" is checked
