I've done the same, though I never used it as a password. Back around 2000 or so I was experimenting a lot with hardware configurations and I had input my Windows 98 SE key so many times during reinstalls I ended up memorizing it unintentionally. Even today, nearly 20 years later, I can recall it perfectly. It actually came in handy a couple of years ago when I built a P-III retro gaming machine out of scavenged parts; I found a Windows 98 CD at work collecting dust on a shelf, installed it, and instinctively entered the correct key without missing a character.