You can use the built-in time synchronization for Windows XP. I'll assume you know what i'm talking about. I think it defaults to updating the time every 7 days, but you can change this setting from within the registry. Let's say you want to change it to update the time daily. Open regedit, and go to the following key...
Find the value that says SpecialPollInterval. Now there are 86400 seconds in a day, so double click the key, change the Base to "decimal", and input 86400 as the value. The time should now update daily. You can choose your own time in seconds if you like.