(GoogleFood: Not posted for reading – just to make life easier for anyone searching for how to do this – probably me next time I reinstall.)
Windows XP Syncs your computer’s clock automatically with Internet time servers. By default, though, it only does it around once a week. you can make it happen more often with a bit of registry editing. If you don’t know how to edit the registy, probably best not to.
You need to go to… HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient …and modify the value for SpecialPollInterval. It’s in seconds, so 3600 (decimal) will check once an hour, 86400 will check once a day.