Windows XP
Windows XP was actually a great OS, and I still use it extensively in testing on virtual machines. Of course, for my test environment, it did not make sense to install all of the components of XP, so I made sure I only installed the lightest components that I thought I needed.
It was only later that I discovered that I could not add components back in because the Add/Remove Programs dialog would error with "Setup was unable to open information file games.inf". Fortunately, this problem is not that difficult to solve.
