Problem re-installing Windows 98

I needed to re-install Windows 98. So when I tired to install Windows the installation program would stop and I would get “illegal operation error message.” The strange thing about this all is that “ANY” problem that I try to install, it would go through the installion and then I would get a error
message: ” a previous program installation was never completed. You need to restart your computer to complete that installation before running setup. Ok, I have restart my system over and over and tried to install a
new program or a upgrade and I keep getting the same error message.
I am trying avoid formatting my hard drive in order to re-install Windows 98. Any suggestion? I appreicate any help that is offered!