800xA IM Server in windows 7 machine
Older versions of IM was supported on client operating systems (e.g. Windows 2000 Professional).
Today's limitation may have to do with product verification and quality assurance.
The task to certify on all existing platforms and variants (32/64-bit, Home, Premium, Enterprise, Ultimate, Standard, DataCenter, etc) is probably too large compared to the number of sold units.
Most "professional" users of IM would probably want to run it on a proper 19" rack mounted server with UPS in a cabinet with controlled enviroment, etc. Such servers probably come preinstalled with a server operating system.
If you need official support for other operating systems, please have your regional ABB sales representative to forward a Temporary Sales Authorization request for you.