I've lost count of how many PC's I've installed a Windows or Linux OS on. Back when I was the web designer/marketing manager of a call center, I kind of doubled as their IT guy for a while. I was mostly useless when it came to networking but everything else I was okay with. And since their last IT guy had left on bad terms, they didn't wanna take risks and asked me to do a fresh install on all the computers there.
Longest and most annoying night ever. Moving from one PC to another, entering stuff (usernames, passwords, license keys, etc) and waiting for all of them to update.