So this one?
IBM Aptiva 2159 S76, any love for these “Stealth” setups?
There seem to be two BIOS variations (ST and FL) in your screenshot, any idea which one of these is used?
For taking a quick look any old hex editor will do, in a pinch.
I don't know about native Windows tools (I'm a Linux guy) but "binwalk -E" is the quickest way to see which parts of the file are compressed and which are not. On Linux, xxd and strings are also helpful.
Most probably there is some compression, perhaps something lha/lzh-based (that's apparently what AMI and Award use). The devil is in the details, though...the headers will be different than what is used with the standard compression tools.