UPDATE:

Success Eventually I pulled out one 256MB chip of my DDR RAM out and started the game. No crashes I managed to finish the game on full settings in the game (1024x768, medium sound (high gets stupid cracks), fauna/flora on, units huge etc.

Tried several times. With both chips in, crash happened 5-20s after starting battle. Sometimes it even caused restart of system.
Without one chip game works perfectly. I tried so far only two battles (Banockburn and Hastings) but will write again if something else happens.

Hey you techinical guys out there. What's the matter with a program not handling properly 512MB of DDR RAM? What could be the reason. Any ideas how to fix this? Any BIOS setting? I tried fault safe settings though and game still crashed with 512MB.

Anyway I am happy it works now. Maybe you could try the same solution Cromwell?