Well I've backed up my databases and other shared folders onto another hard drive and turned the server off.
This is a good time to get your vacuum cleaner out and give the box a good going over. Especially the inside cooling fans and heat sinks.
This box seems to be more dusty than I would have expected. And it is quite hot to touch. I might have to rethink the server location.
Ok, Choosing a New Installation. I can now confirm that the installation WILL format all attached hard drives! This is important to note as had I not disconnected the backup hard drive it would have been up for formatting.
Estimated time for installation: 50 minutes.