In Windows XP you can get to the 'Internet Time' settings (to select an NTP server or force an update) via the 'Date and Time Properties' in Control Panel.
Where is this found in Windows 7?
As pointed out, I won't be able to do this if attached to
a domain. My problem was that my PC was trying to synchronise with 'time.windows.com' which is blocked by our firewall. In the end I changed the registry key (
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\DateTime\Servers) to point to our internal NTP server and the time is now synchronising fine.
(I know that domain computers should synchronise with their domain controllers, but this machine is not actually logged in to the domain most of the time.)