C:\WINDOWS\system32\cmd.exe "C:\Program Files (x86)\GOG.com\Ground Control and Expansion\" /c start /high /affinity 01 gc.exe
and
C:\WINDOWS\system32\cmd.exe "C:\Program Files (x86)\GOG.com\Ground Control and Expansion\" /c start /high /affinity 01 gc.exe -mod DARKCONS
on 64-bit windows,
C:\WINDOWS\system32\cmd.exe "C:\Program Files\GOG.com\Ground Control and Expansion\" /c start /high /affinity 01 gc.exe
and
C:\WINDOWS\system32\cmd.exe "C:\Program Files (x86)\GOG.com\Ground Control and Expansion\" /c start /high /affinity 01 gc.exe -mod DARKCONS
on 32-bit windows, to start the original and the expansion, respectively.

Oh, I used '01' above but in fact, I actually use '8' on my system and you almost certainly want to use something other than the first core of your multicore/multiprocessor system, mine's a quad-core Phenom II so I use the last (core 4) I'd recommend a similar approach, especially as on my system, when experimenting, setting it to the first core DOES NOT improve things and actually makes it worse, I reckon.
You need to be careful with the numbering because it might select a combination of cores rather than a single one if you don't understand the way it works fully (it's a bitmask for those who know what that is) but '2', '4' and '8' should all work fine (the latter if you actually HAVE four, of course). Also bear in mind that 'hyper-threading' is counted for this purpose (and by the game, which doesn't run well on load-spreading multicore/multiprocessor systems), so a hyper-threaded dual-core CPU has four cores for this purpose.
It was late when I posted the original thread starter, so here is a bit more information:
This is a commandline which increases the priority of the application (Ground Control, in this case) and sets its affinity to a single core (makes it use only the selected core), it could easily be edited for use with other applications, if you like. I recommend its use because, like many older applications, especially games, GC is sensitive to timing issues that arise from the use of multiple core with variable clock speeds, also increasing its priority doesn't hurt one bit.
I put graphics to maximum and forced my drivers to use maximum anti-aliasing and anisotropy, then observed a doubling in framerate (and hugely improved response to mouse, etc) with the affinity corrected, the improved scheduling prioirty added another couple but it was basically a 'might as well' addon.
I hope this helps and feel free to contact me help with this or similar situations with other games.
P.S. The leading zero isn't needed, I have discovered and I also edited an earlier mistake which would have led to confusion about the mask and what numbers select what. Everyone can safely use '2', in any case, if their machine has multiple cores or processors.