XP not able to use pool.ntp.org timeservers

[#1]

It appears that my XP Home SP3 system cannot use any of the pool.ntp.org servers for time sync -- it always errors out with "An error occurred while synchronizing with ..."
I've tried pool.ntp.org, europe.pool.ntp.org, uk.pool.ntp.org and so on ...
If I use time.nist.gov or time.windows.com it works ok, as it does if I use ntp1.npl.co.uk
Googling the problem hasn't so far produced any hits specific to pool.ntp.org for this error
(I realise there are other time clients and command-line options, so don;t need advice on that front.)
But I did wonder if anyone else had had the same problem with ntp.org and Windows and had found out why?
paul

[#2]

I know this won't be much help but I'm using uk.pool.ntp.org in both a Windows XP SP3 laptop and a Vista desktop without problem. 
Have you tried turning any firewalls off temporarily?
Edit: Edited to make sense!!!

[#3]

thanks Paul ... ah yes the obvious thing I neglected to try first ...
(I was thinking it can't be a firewall problem because other servers work; i really don't learn from experience)
Yes, turning ZA off allowed sync to uk.ntp.pool.org
Looking at the logs it seems incoming connections from ntp.org are being blocked.
But none of the other timerservers are being blocked.
Odd

[#4]

Why would ntp.org need to create an incoming connection with your machine?
Surely your machine should be making an outgoing connection to ntp.org ?

[#5]

UDP?
B.

[#6]

Quote from: Barry

UDP?

Possibly, many (most?) firewalls these days have a time based UDP state engine so if the pool.ntp.org servers are a bit slow in replying it could be seen as an incoming connction. Shouldn't be a regular/ongoing problem though.
Mike

[#7]

Quote from: Barry

UDP?

Yeah, port 123, but the connection should always be initiated by the local computer requesting the time [ack. simplification of NTP].
The remote computer (in this case ntp.org) should not be attempting to initiate any connection - it should simply provide a response to a request made by the local computer.
As the request is coming from the local computer, the firewall should be aware of the request going out, and should not see the incoming response as a new connection.
Since this works fine with other ntp sources, what is it that ntp.org is doing differently that requires it to initiate a connection?
Is ntp.org attempting to connect to UDP:123, or is it another port it wants to connect to?

[#8]

At work today I was reading about the Windows W32time service.  It seems that it's designed to be "good enough" but not accurate to within about 20 seconds or so.
I also maintain some systems where the time has to be spot on (at leats to the nearest half-second, and I use a free program called AboutTime.  This makes the same NTP calls, but it also pings the time server to measure the comms latency, then corrects the response from the NTP server to make sure the computer time is set taking account of the delay.  Worth considering, as it seems to be more accurate.