The first determination is whether your operating system is eligible for upgrade. In Blake Handler's blog post, he provides an easy-to-follow documented list of both the supported as well as unsupported upgrade paths for editions of the Windows 7 operating system. Note that if you have Windows XP, it will be necessary to do a clean install.
An absolute "must read" is Ed Bott’s article, "Windows 7 Upgrade FAQ", at Microsoft Report, ZDNet.com. Included in the FAQ is a nice explanation of the difference between an OEM license, an upgrade license, a full license, and a volume license.
- Blake Handler: Microsoft Documents "Windows 7 Upgrade Paths"
- Ed Bott: The Ultimate Windows 7 Upgrade FAQ
Remember - "A day without laughter is a day wasted."
May the wind sing to you and the sun rise in your heart...