The easy simple answer is: Macs feature different hardware chipsets than PC's.
The better answer.
Macs use a Motorola - IBM jointly developed processor called the Power PC chip. They are based in IBM's POWER chip set that was started in the 1990's. PC's use chips based on Intel's 286 processor, although the Pentium and later chip families enhanced the capabilities of the processor.
The older Macs -- talking early 1990's -- used NuBus technology developed in Texas (I think it was Texas Instruments) for cards. Windows at the time was working with ISA technology. MCA was being phased out, as it was too expensive / proprietary for the Windows crowd. I should clarify -- the DOS crowd. Macs used SCSI busses (more expensive, but more capable) to access hard drives and scanners. PC's used the parallel port and IDE drives.
In the Mid 90's, Intel developed the PCI bus. They first came in Windows machines, and then Apple adopted the technology into the later PowerMac series before the G3 chip. Apple also got into internal IDE drives. USB came to both platforms later on.
In the 2000's, Apple developed Firewire that was adopted by Macintosh and Intel Platofrms.
So today's hardware is similar in terms of internal busses and expansion options. I can go to Best Buy and get a IDE hard drive, and place it into a modern Mac and format it and use it. A PCI network card, with the proper drivers, should be cross platform. I am not certain if video cards are supported cross platform -- many are not, but some might be. Apple's laptops do have a VGA video out, although we are seeing more S-Video outs.
But the heart of the computer -- the CPU -- remains very different. They access memory in different styles, they run different instruction sets. The PPC chips (G3 G4 G5) are very RISC oriented. The Pentium series are more CISC oriented. Both chips are getting deeper pipeline technologies, and branch predictions are continued to be developed.
This discussion was not meant to be exhaustive... others are welcome to it.
Marklar is the Mac OS X compiled on x86 hardware. It is not available to the public, nor I expect, will it ever be. (Publicly available) Mac OS X knows nothing of PC hardware and cannot be installed on FAT/NTFS volumes. Sorry buddy, you're stuck with Windows, Linux, and/or a BSD variant.