Microsoft Windows is a term for a family of operating systems produced by Microsoft, and these are the only serious competition for Linux on desktop machines (MacOS is limited to proprietary hardware and as such doesn't really compete).
Linux and Windows Inter-Compatability
- Samba a suite of software enabling UNIX and Linux systems to both act as clients accessing Windows PCs and as servers for them.
Open Source and Windows
Although Windows is not a Linux distribution, it may be worth while to list Open Source programs that are available for Windows, since there is a very strong overlap (almost one-to-one) between the Open Source and Linux communities. Also, many Linux programs have been ported over to Windows. Introducing these ported programs to Windows users may ease any future migration to Linux. In spite of less protection, given by this OS, it is still more popular among the common users. However, Linux's usability is more suitable for the computer network architects 
- ReactOS is a project that is attempting to do an Open Source reimplementation of a Windows-like operating system. It is currently in alpha, and is about 50% complete.
If you have ever bought a computer that came with Windows installed, but used Linux instead, you have the right to a refund. Read the EULA and you will find "If you do not agree, do not install or use the product; you may return it to your place of purchase for a full refund". Microsoft has to offer this because technically they sell licenses (not software) and a license cannot be accepted by simply making a purchase - you have to click the "I agree" button to be bound by it.
If you want to buy a computer with Linux pre-installed, see Pre-installed Linux