Microsoft doesn't sound too confident in their upgrade process if you aren't running Vista:
* Windows Vista
The transition to Windows 7 should be pretty smooth, although you may need to update your system to the latest service pack first.
* Windows XP (or any other operating system)
We recommend that you experience Windows 7 on a new PC. While we don't recommend it, you can opt to upgrade your current PC from Windows XP or another operating system to Windows 7. We recommend that you get help with this process from your local computer service provider. You'll need to back up your current files and settings, perform a custom (clean) installation, and then reinstall your files, settings, and programs.