Real late to this party, but while we're all having problems installing Protel on modern systems:
Anyone have experience installing Protel 99SE fresh on Win 10? Every time I try, it messes up the environment variables and windows loses its mind. A reproducible error is right-click my computer, properties, click "Advanced system settings":
It errors with: "Windows cannot find '%SystemRoot%\system32\systempropertiesadvanced.exe'. Make sure you typed the name correctly, and then try again."
Odd thing is, %SystemRoot% IS properly defined as C:\Windows.
Haven't even gotten to the point of trying the fixes above, though I've tried opening a .PCB and it didn't go so hot, so that's next.