How can I install boot camp if I have a Windows 7 upgrade CD?
Actually, it's not that Apple won't let you install Windows from an upgrade disc—it's Microsoft that won't.

Think of it in terms of a blank hard drive on (what will be) a Windows PC. You can't use an upgrade disc to install Windows, as there's no OS there to upgrade.

However, once Windows is installed, you can then upgrade to a later version of Windows. This is covered in the Boot Camp
Installation &amp; Setup Guide (pdf), page 15+.