Windows has a built-in NTP server
This was painless. I was looking for a free NTP server for Windows, and it turns out there is a built-in time server on XP and Win2k.
How to enable a local Windows NTP server by modifying the registry The site appears to be useful, despite the copious amount of ads on it: Windows Registry Guide
I feel like I have discovered hot water...
(Edited at 8:20 pm on the same day) Apparently the above is not a reliable guide. It appears to be accurate for Win2k, but the story for Windows XP is not exactly the same. Using Windows XP Professional with Service Pack 2 in a Managed Environment: Controlling Communication with the Internet seems to be the reliable guide for Windows XP. I will report progress with this later - the goal is to have every machine (mix of WinXP and Win2k embedded) on a domain-less Windows network synchronize its clock with one specific (WinXP) machine on the same network. It is OK for the time to be incorrect, it just has to be relatively consistent.