I've never used either of these, so I can't really recommend one, but both Turbo (freeware) and CPUKiller (shareware) were designed for Windows. You can read a little about these at
http://www.sierrahelp.com/Utilities/SlowdownUtilities.htmlHowever, note that their download link for Turbo is old and thus broken. It should be adjusted to
http://www.oldskool.org/guides/oldonnew/resources/turbo.zipSomeone else here may have a better suggestion for you if these don't work. I assume you've also looked up the specific game and made sure there isn't a patch or other workaround which you can do to reduce the speed?
Good luck,
-Luke