This issue used to be reported fairly frequently, but not recently. Have you got a checkmark against Network Time Protocol in the firewall section in Security Center which really shouldn't be necessary as system services are silently allowed by McAfee?
There was also an issue in Vista with network time not synching but that was supposed to have been dealt with ages ago - make sure Vista is up to date with all updates whether critical or not. Have you tried using a different time server when you call up the System Clock?
Also make sure that you don't have Windows Firewall turned on, or any other firewall for that matter.
Also advise if you are behind a proxy....such as those found in Universities.