First, check that the W32Time service is enabled and running. There is a known issue when Windows Server 2003 SP1 has been installed without also installing SBS 2003 SP1. See Service Packs and Windows Updates for SBS.

net time /setsntp:ntp.internode.on.net
(do that once.)

IP Packet Filter:
Name: NTP (arbitrary)
Filter Type:Custom, UDP, Send receive, local:all ports, remote:fixed port,
123

If you want to test, net stop "windows time", net start "windows time", "w32tm /resync /nowait", then check System Event Logs.

If you want to check where it's currently pulling time data from, type the following command: net time /query

Alternatively, Susan Bradley suggests these steps:

w32tm /config /manualpeerlist:pool.ntp.org /syncfromflags:manual
w32tm /config /update
net stop w32time
net start w32time
w32tm /resync /rediscover