Prior to Vista a lot of code was written on the assumption that the process would be run with administrator privileges. While this wasn't the original security model for windows NT, 16-bit windows, and windows 95 et al didn't have a security model as such, so code that had been written for those operating systems was particularly problematic.
It was definitely the case that a large number of windows users, possibly a majority, would have their user account be a member of the Administrators group.
UAC changed the way processes were started to use a lower privilege level by default, or popup a permissions dialog if higher permissions were needed (based on a manifest associated with the process)
In parallel the code samples on MSDN that showed how to detect if the current process had admin permissions was subtly incorrect, so programs that had been written to do the right thing depending on privilege level would fail unexpectedly on Vista
It was definitely the case that a large number of windows users, possibly a majority, would have their user account be a member of the Administrators group.
UAC changed the way processes were started to use a lower privilege level by default, or popup a permissions dialog if higher permissions were needed (based on a manifest associated with the process)
In parallel the code samples on MSDN that showed how to detect if the current process had admin permissions was subtly incorrect, so programs that had been written to do the right thing depending on privilege level would fail unexpectedly on Vista