For those of you having problems running the game, try the following:
(you might want to disable your net connection before you try this)
Click start -> run, type msconfig and hit enter
On the new window, click startup tab.
In the box that appears, click disable all if you have XP. If not, uncheck everything but explorer, systray, loadpowerprofile and scanreg. Click ok
reboot when asked. If msconfig opens again after the reboot, cancel (don't reboot).
Try the game now.
If it works, something in the background is stopping M&B running. Re-run msconfig (you'll need to do this anyway). Go back to the startup tab. Everything listed there is a program running when windows starts up. It should be obvious from the name what the program is. If it is something you don't need or recognise, leave it unchecked. You'll shorten the load time for windows and improve system performance if you leave everything out but your antivirus and firewall.