How to slow down my CPU?

Have you tried using the built-in power options in Windows 7? I think this is achievable by choosing a passive cooling strategy.:

1. If you hover your mouse over the little power icon in the activity bar, you should see this:

2. Click "More power options", and this window is shown:

3. Choose "Change plan settings" to get here:

4. You should find what you're looking for under "Change advanced power settings":
   The tooltip is from hovering over "Plugged in: Active". If you hover over "On battery: Passive" you see the text Slow the processor before increasing fan speed, which sounds like what you're trying to do.