Ed Bott just uploaded a very timely post about the Windows 7 upgrade issues that have arisen this week. You may recall my previous posts and article in which I have tried to sort through the mess of how it is you can clean install Windows 7 with Upgrade media. Ed takes it further, however, and if you're wondering about what's going on or struggling with your own upgrade, this is a must-read:

What's the difference between the full and upgrade versions?

It’s all about the product key. When you enter the product key, the setup program checks to see whether you installed the product on a clean system that didn’t previously have any version of Windows installed. If the answer is yes, it blocks you from entering that key.

What happens if I try to do an install without a product key, then add the product key from my upgrade package later?

It fails.

How about 32-bit to 64-bit upgrades?

You can’t run the 64-bit installer from an existing 32-bit Windows installation (or vice-versa). Here’s how you have to do it instead. Start your computer using the 64-bit installation media. When prompted, choose the Custom installation option.

There is a lot more. Be sure to read this full post.