Windows rot happens if you install/uninstall a lot of programs due to DLL hell. I suspect I have to reinstall way more often than my parents, but it does happen even to them.
And even then it's probably only certain applications. Office plugins seem to be among the worst offenders in my experience. I remember problems with both IBM Rational and FileNet Panagon. Those are both huge software packages developed by big companies, so I don't know how much blame to lay on microsoft.
Barring hard drive issues, I've never had to reinstall the OS on my personal PC.
