I'm running two Win2k3 DCs, one Exchange 2k7 box, a Blackberry box, a file/print server, Sharepoint, WSUS and ~40 XP and Win7 workstations. I've discovered that our network is ~3 minutes ahead of ...
IIRC windows doesn't run ntp by default, it runs some propretary w32time service instead. I think it has an ntp client built in, but I'm not sure if it can be an NTP server out of the box.