Yes,
Apple has a set of Windows drivers and software called Boot Camp to make Windows "see" the Apple hardware.
As of 10.7 Lion you can only use W7 and above. Apple no longer supports, nor has drivers, for XP or Vista on Apple computers that came installed with Lion. Basically from late 2011 on.
Windows is an operating system produced by the Microsoft company. Windows is not a computer or a company. The first Apple computer was sold in July 1976. The first version of Microsoft's Windows operating system was sold in 1985.
No. if you have an apple computer you can have windows xp but windows xp computer can not run apple sorry dude
No it is a Windows PC. Apple computer run the OSx operating system which is currently called Snow Leopard.
Most Apple software is designed to work on their Mac OS X operating system and will not work on a computer running the Windows operating system. Apple does make some of their software available for Windows such as iTunes and their Safari browser.
Windows is the operating system on a Microsoft computer or an Apple computer running windows drivers. Without windows your computer would not work.
Windows is a popular operating system, but it is made by Microsoft, not Apple.
An operating system (OS) is the software used to run a computer. Windows is an example for a operating system. Apple designed the Mac OS to run more stable.
The 2 main operating systems using on computers are Microsoft Windows and Apple Mac OS. Windows is available on all PCs and Mac OS is available on any Apple Computer.
Apple's Macintosh computers use Apple's operating system known as OSX. The current version is OSX 10.8.#. Macintosh computers can also run the Windows and Linux operating systems, but those systems are not included in the purchase of the computer.
Windows is an operating system developed by Microsoft. Apple makes Macintosh computers that run Apple's own Mac OS X operating system but can also run the Windows operating system.
Measuring the market share will depend upon what market you are looking at and how you are measuring the size of the share. Bearing in mind that Windows is an operating system that can be run on an Apple computer and Apple sells both hardware and software: 

One way is to monitor web browsing to see the percentages of different computer platforms. Such a measure gives the Mac around 10% of the market and Windows around 90% (See links below) 

If you define the market as the quality end of computer sales ($1000 and up), which Apple targets, then Apple has around 91% of the market. (See links below)
There are many types and brands of computer operating systems. Some of them include Windows, Apple OSX, Ubuntu, UNIX, Linux, and Android.