Try the following:
1. Find out your computer spec, especially CPU (Pentium II, III, IV or AMD) and speed (100htz, 333htz, 500htz etc), amount of RAM memory (32,64,128,256,512? and graphics card. You can find this info when you start the system and also on my computer. What make of computer is it?
2) Compare you system spec with the game requirements (which are on the case). You may need more RAM or something.
3) When computer is rebooted, press alt, control and delete together and tasklist and see what is running in the background. Close all but systray and explorer.
4) When in game, go into setting and change graphic details -e.g. do not show shadows etc. The more detail the slower the game.