With reference to reinstalling Windows... In the NT line, I've found it to rarely be necessary. When I was doing full-time tech support / computer repairs, I'd say maybe two or three times a year I'd come across a Windows system that was so hosed that I had to reinstall it. The rest I could fix with varying amounts of time and effort.
Whether they were worth fixing or not was another question. If a customer had a heavily infected machine with little or nothing of any importance on it, sometimes a reformat / reinstall was the cheaper, more expedient option. But it was just that: an option. In the vast majority of cases, the systems could have been fixed.
With my home computers, I have all data that I care about stored on a file server with redundant storage. I have installers for all my must have apps on there too... so I've sort of streamlined the "nuke and pave" option for myself. I've gotten to where I just don't take more than maybe 30 minutes of prodding before I just decide to start over clean.
I don't do tech support professionally anymore but sometimes a friend leaves me with a machine... or I barter for something. Usually these are 3+ year old machines that take 5 minutes to boot up. Conversation usually is something like...
"Ok, is there anything on there you need off it? "
"Just some music but I don't really care. Do whatever you need to."
*copy files... or image entire disk if I have lots of time*
*nuke & pave*
"Oh wow I can't believe how fast it is now"
Of course Ubuntu used to beat all of them before they went to the Unity desktop. That OS would go upgrade to upgrade not only without a reinstall but often without a reboot. But boy oh boy I can't stand Unity