phyneas :
When you say a clean install, you mean you are re-installing the same operating system, or installing a different operating system? In either event you don't need to deactivate or unregister anything, at least that I can think of, unless you are referring to programs that limit the number of machines that a certain license can be authorized to use, like an itunes profile or certain DRM games. I don't know what programs you have or which of them would require that, or whether when you install the operating system and then reinstall them they would automatically recognize the machine as being the same one. In terms of just installing an operating system, no you don't need to deactivate or unregister anything to accomplish a totally clean install, it will do that for you just by installing the operating system. Depending on which OS you are on and going to you might even be able to transfer your files and settings and programs as part of the install if you want to - I believe that current versions of Windows allow for that. The Windows license itself can only be used once at a time so by reinstalling it you are wiping out the old license so you can re-enter it normally. Remember as well to have all of your drivers ready before you install the new operating system so you can input them once the install is done.
I was having hardware and some software trouble (computer won’t let me uninstall somethings that got downloaded trying to fix the hardware problem) so I wanted to just wipe my drive totally clean and reinstall windows 8, and maybe a dual boot with win7.
Programs like PD11, Vegas movie studio, starry night pro.
Posted my trouble with hardware no one has replied yet.
( http://www.tomshardware.com/answers/id-1907462/application-found-fix.html)
And thanks that answered some of my questions.