It might be the amount of programs installed, which can decrease performance. It might be your specs(it could be a possibility, but I doubt this for now).
If you use Windows 7, Vista, or XP, switch to the Classic theme. It can really boost performance. This is how to do it: go to Computer, or My Computer, right click it and click "Properties". Go to "Advanced system settings" on 8, 7, or Vista, or "Advanced" on XP. Go to the "Performance" menu and click "Settings". Stay on the "Visual effects" tab. Click the button that says "Adjust for best performance." Click Apply, and then OK. You can X off all the windows open, and you will see how your performance boosts.
I hope this helped. If it didn't work, please contact me.