Thank you for your post.
One of my machines has Windows XP, and I have FileMaker Pro 9, 8.5, 8, and 6 all installed on my hard drive. What happens when you try to install FileMaker Pro 6 Developer? What error message is displayed?
Inside your Program Files folder, there should be a FileMaker folder, and inside that, there should be a folder for FileMaker Pro 6 Developer.