Could adding Windows 7 to my MacBook be harmful?

No, it will be not harmful to your Mac, as using Windows will not affect Mac OS X, be it running Windows separately via Boot Camp or as a VM (see below, though you need RAM for this).
Booting Windows on the Mac

You should also backup your Mac OS X side before installing Windows, in case the installation goes wrong.

Upgrading RAM in MacBooks and MacBook Pros​

MacBook, MacBook Pro: Replacing the Hard Disk Drive, transferring data to the new HDD​

How to format a new HDD/SSD to install Mac OS X or make a bootable copy onto​

Also have a look here, to find out, what is making your Mac run slow:

Performance Tips For Mac OS X by GGJstudios​

 

Which Mac do you have. What year is it and what are the specs on it?

 

It's a 2007 MacBook, thus I guess it has a round 2.13 GHz and at least 1 GB RAM.

 

Depending on what version of Windows you have to install, you might have to upgrade your RAM. The 64 bit versions of Windows 7 require 2GB; I'm currently doing the same thing to my 2009 model MacBook. My school requires me to have Windows and I really don't want to give up Mac OS X so this is the best option for everybody. I'll let you know how my installation goes when it's all completed. From the looks of the instructions on the internet it shouldn't be hard at all.

 

Windows XP is the best option for you. It will run under 1 GB ram. Vista and 7 need a lot more to run anything.

 

Adding Windows 7 to your mac wouldn't make your computer slower. If it is slow, you should try to add more RAM.. That usually helps with speeds.