Congrats on switching.
You might be able to use XP from an old laptop.
Two potential issues:
1. Some laptops come with "system restore CDs" that will only restore Windows to the laptop that the CDs came with. As long as you have a real Windows XP install CD, that shouldn't be an issue.
2. If you're going to use Apple's free Boot Camp utility (which lets you choose which OS you want to boot into when you start the computer) to run XP on your MacBook, the Windows XP install CD you use needs to have Service Pack 2 included.
If your laptop is old, the XP CD probably won't have Service Pack 2. If you have access to a Windows computer (and a CD writer), there are instructions that you can follow to burn your own XP SP 2 install CD.