Well, if I'd done a bit more research, I might have found this at
Tweakguides.com:
Ubisoft, the distributors of Dark Messiah of Might & Magic, have teamed up with Valve, the developers of the Source engine upon which Dark Messiah has been developed, to allow the game to be distributed via the Steam online distribution system as well as through normal retail channels. For more details of how Steam works, and to troubleshoot any Steam issues you may have see the Steam Support Page.
The Steam application is required to run whenever you run the Multiplayer aspect of Dark Messiah. However if you purchased a normal retail DVD copy of Dark Messiah, you don't have to run Steam at any time to run the Singleplayer aspect of the game. But there is a bug in the retail version of the game such that if you install the multiplayer component along with the singleplayer component of the game, after installation Steam will launch and commence redownloading a major portion of the entire singleplayer portion of the game again. As mentioned in the patch section above, I recommend that you do install Steam and allow it to update your singleplayer game files as these updates are necessary, however to save it from redownloading existing files, follow these steps:
1. Close down Dark Messiah and close the Steam client so it is not displaying in your System Tray.
2. Go to your Program Files\Ubisoft\Dark Messiah of Might and Magic\mm\ folder and highlight and copy all the .gcf files.
3. Go to your \Program Files\Valve\Steam\SteamApps\ folder and paste these files there.
4. Restart Steam while online - Steam will now update your Dark Messiah singleplayer component more quickly as it needs to download/update fewer files now.
In general it is recommended that you allow the game to be updated by Steam as often as possible. You don't need to keep Steam running all the time to do this, but make sure to check this page for notification of major updates, and also regularly restart your Steam client while online to see if any updates are required - they will download and install themselves automatically if any exist.
As a final note, make sure to defragment your drive after updating via Steam, and to make absolutely certian that all your game files are valid and working properly, in Steam right-click on your game name under My Games, select Properties>Local Files>'Verify integrity of game cache'. This will run a check to make sure all your game files are sound, and that nothing has been corrupted.
Oh well, no big deal. At least I won't have to worry about keeping the game patched now...