Windows Server unable to synchronize NTP time reliably

Why does Windows Server (2008, in this case, but I've seen the same problem in 2003) seem to have problem synchronizing time? I've seen this error in my System log across a variety of servers:

The time service has not synchronized the system time for 86400 seconds because none of the time service providers provided a usable time stamp. The time service will not update the local system time until it is able to synchronize with a time source. If the local system is configured to act as a time server for clients, it will stop advertising as a time source to clients. The time service will continue to retry and sync time with its time sources. Check system event log for other W32time events for more details. Run 'w32tm /resync' to force an instant time synchronization.

Under Control Panel, Date and Time, The Internet Time Settings are set to synchronize with time-nw.nist.gov ; the last successful sync was 2 days ago, indicating there's some kind of problem. But if I click the "update now" button on that dialog, indeed, it updates with the time!

So why can't windows server reliably time sync via NTP in the background without me manually intervening? What am I doing wrong?

From serverfault Jeff Atwood

• Jeff, I found this article. Might be of some help to you. You might have already read this but I thought it was worth a shot.

  HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Config\AnnounceFlags
  

  This registry entry controls whether the local computer is marked as a reliable time server (which is only possible if the previous registry entry is set to NTP as described above). Change this REG_DWORD value from 10 to 5 here.

  UPDATE As per Jeff, best source of reference is http://support.microsoft.com/kb/816042

  Jeff Atwood : weird, so you mark it as "not reliable", lol. OK, I made this change, let's see if it helps.

  Joseph : Glad I could be of help!

  Duncan Smart : KB 816042: "... and uses the built-in complementary metal oxide semiconductor (CMOS) clock" - don't you just love technical editors?
  From Joseph

• Another article

  From Justin Love

• When synchronized time is important, I use NTPD for windows

  http://www.meinberg.de/english/sw/ntp.htm

  I install it, point it at north-america.pool.ntp.org and forget it

  From Mark Porter

• Windows Time Agent is a free control panel applet for configuring the NTP server/synchronization ability of Windows. It acts as a front-end to the registry settings and lets you configure multiple NTP servers and see what sort of results you are getting from them in real-time.

  Not many people know about this particularly handy (and free) piece of software, but it's a must-install on servers I maintain.

  paracycle : Thanks a lot mate. Windows Time Agent is a gem. It just became a must-have install on all the servers I manage as well.
  From crb