on a few windows xp (so far) the windows clock can alter itself for no reason. It seems to follow that once a day it reverts by one hour, i think this is something to do with summer / GMT time. The automatic time sync does not work either. It still maintains an hour difference. It is not bios related.

Try the following options

• change the time sync server and try a new sync
• disable firewall on router and try sync. I have found that a router for no reason can block the time sync but once it has allowed it there seems to be more issues
• Try this daylight saving patch
• Time keeps changing by itself in Windows XP
• February 2010 cumulative time zone update for Windows operating systems
  Some time patches here from microsoft
• an error occurred while windows was synchronizing with time.windows.com