I assume your system type should be 64 bit OS… Tell me your wM 8.0 image is it 32 bit or 64 bit…? I am not sure you can install it on Windows 8/8.1 OS… You can give a try 
As per the system requirements guide:
webMethdos 8.X can be installed on Windows 7 Professional, Ultimate, Enterprise Edition
webMethods 9.5 SP1 and up can be installed on Windows 7 Professional, Ultimate, Enterprise Edition, Windows 8 Pro and Enterprise
I suggest you to refer the System Requirements PDF specific to webMethods version.
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