C2K5 gameplay problem

[ahk2578]

I haven't searched the threads so pardon me if this was raised before: I have noticed quite recently that my game shuts down and returns to the desktop after a dismissal and the replay of the dismissal is in progress......can you guys explain why this may be happening....specs for my PC are 20Gb HD (50% utilised ,256MB memory , 64Mb video card ....AMD athlon processor .....I can't remember the processor speed !!
thanks for your advice !!