Yesterday I bought Windows 8 Pro (an upgrade version as I found out later) and attempted to install it using the weird little utility I downloaded from Windows.com. Installation kept failing with vague errors, so I loaded 8 up on a USB drive I had and did a clean install on another HD, which worked fine.
However, I couldn't activate it because it turns out the version of Windows 8 you get from Windows.com is the upgrade version, which sucks. So you have to upgrade from a previous version of Windows.
Did some googling and found out I could do a clean install, I just couldn't format the disk. All I had to do was to choose the current Windows 7 partition as the install destination during setup, and voila it worked. Activation worked just fine, woo. And I was able to easily delete my Ubuntu partitions and extend the Windows one, which is awesome.
I just wish I could have kept my installed programs, it's a pain in the ass to install all of those again. But I'm pretty happy with Windows 8 besides that so I guess it's not that big of an issue.