The most powerful computer on a modern passenger airline is the in flight entertainment system. Seriously.
Aircraft FMCs, FADECs and electronic flight control systems are not performance systems, at all. Clock speeds in comparison to modern microprocessors are Slooow, and there isnt particularly much RAM. Note, there are rarely Hard disk drives, as unless they are sealed, typically fail above 20,000' as the read head relies on an air gap to avoid contacting the disc.
Microsoft? I take it you dont work with Aircraft. The 'operating systems' consist of multiple redundancy lines of code that are optimised for reliability and we dont use any recognisable operating system on modern aircraft. For example, all a FADEC does is map parameters to a fuel flow and inlet guide vanes; it is not required to send email!