1. Do you have an antivirus running in the background? If so, it may be lagging your game (more common with single core processors)
2. Press control, alt, delete while you are playing Majesty 2. What percentage of your ram and processor power are you using while the game runs? If they're close to 100% look to see which programs are using up your resources and close them. Especially keep an eye out for malware.
3. Have you scanned your computer recently for all types of malware, viruses, rootkits, etc.? If not, do so.
4. Are all your drivers up to date? This includes any USB device currently plugged into your computer.
5. Is your hard drive running out of room and have you defragged in the past month? If not, do so. Try to not fill your hard drive to full capacity. I prefer less than 50% but under 80% should be fine.
Basically look for anything that could be stealing your power while you play majesty 2. Make sure that when your computer is idle you are barely using any of your resources. This way you can rule out common problems that may be lagging your game.