Then I found this page, which has some neat tips about adding new time servers, and (more importantly) making the automatic syncing more frequent. After changing the registry value (it mentions to a smaller value like 60; make sure you aren't in base Hexadecimal), and logging out or restarting, Windows will sync the time more frequently, correcting it when it goes wrong.
It's not the best solution, and the problem still occurs, but it shouldn't be noticeable anymore.
[robg adds: No, we won't regularly run Windows hints, but as noted on the linked tip, if they seem general enough and relate to the basic system, we will -- there are a fair number of Mac users who do run Windows on their Intel Macs, and this information can be of use to them. Note that we've only run a handful of such hints since the Intel Macs shipped, so there's no fear of the site's focus changing! :) ]

