I had the same experience with Windows XP growing up. Very few months I would reinstall my desktop computer that ran WinXP, and I did the same with the laptop I had after that which was also running WinXP. During that time I used to install a lot of random freeware and shareware and trial software and games and game demos. And probably those things were leaving stuff behind like you guys said.

The very first time I reinstalled WinXP was shortly after I got my first computer of my own for my 12th birthday. I was tinkering with some of the files, and discovered that I could open .cur cursor files in MS Paint. I edited one of the cursor files, saved it as a bmp and renamed the file to .cur under the assumption that this would work. Set my custom cur file as cursor and logged out. When I tried to log back in I couldn’t. Whoops. Freaked out for a second thinking I had broken everything. Remembered that there was a red floppy disk with an ambulance on it that came with the computer. Rebooted the machine with the red floppy disk and reinstalled Windows. Lost my files but had nothing valuable at the time anyways. Have lost more valuable data since then. And expect to lose more data in the future still :p