Hey Cobra!
I have Lock On: Air Combat Simulation, (Its the first version released).. Its a cool dogfighting "flightsim". But i never play it anymore, i kind of got bored with it when my old PC was not up to its standard. It kept shutting down on me and acted wierd, (the game that is). But this was because i did not have a sufficient graphicscard. You do need one graphicscard with a minimum of 128Mb and at least 512Mb RAM.
I dont know the specs for Modern Combat, but i do belive you atleast need the same or a bit higher.
Celeron Windows XP 256GB RAM You probably mean 256Mb RAM.
But like your cousin says, you can downgrade the graphicsettings under "options". Just test you way forward and see what works best. Then if you are not satisfied with the graphics, you can upgrade the hardware.