What you have is an RS/6000, not a PC. It uses the PowerPC (or Power or Power2, depending on the age of the system) processor and not an Intel x86 processor. It will not boot off of your standard PC floppy disks and will not run an operating system compiled to the x86 instruction set. Also, as a security procedure, it will not boot off of any external device (floppy, CD, tape) unless you either put the key into service mode (for older systems) or press a function key (I believe F5 or F8, someone correct me if I'm wrong) during the boot sequence on newer systems.
What are you looking to do with the machine? AIX 5L has Linux libraries so you can compile and run Linux apps on it. I think there may be a version of Linux that runs on the PowerPC platform, but it's been a while since I looked at it.