There's a lot of confusion in this Thread...

Mac is the abbreviation for Macintosh which is a computer (piece of hardware). You cannot replace Windows (Software) with Hardware! You can replace your PC with a Mac (i.e. buy a new Computer) but that's a different story.

The operating system which is running on Macs is called Mac OS or Mac OS X in its latest incarnation.

The most recent Macintosh computers are using Processors from Intel and except for the design they're almost identical to standard PCs (these Intel based Macs can run Windows, that means you can replace Mac OS X with Windows). Apple has taken pre-cautions however to make sure that you cannot run Mac OS X on standard PCs and it's not possible to obtain a legal copy of Mac OS X without buying a Mac.

There's also the option to run Mac OS X in an emulation (like PearPC) but that's painfully slow!