95% of all PCs use a Windows Operating system. About 2-3% use Apple's OS. The remainder uses 3rd-party-programs. The Console gaming is far more equal, with more weight on Nintendo's Wii (successful advertising as a "family" console with "intuitive" controls being the primary reason.) If you're talking a PC, Microsoft has an effective monopoly.
Why else do all PC manufacturers immedately adopt the latest Microsoft OS when it gets released on their pre-builts?
Thankfully, it hasn't been 95% in some time. Windows is around 85-90%, OS X is between 6-10%, and Linux is around 2 or 3%
A monopoly is when one company owns/controls all the pieces of any single market. As long as there's competition that's strong enough to not be squashed out by the bigger dude, and there is in this case, there's no monopoly.
That's the strict definition of monopoly, but in real-world situations if a company has overwhelming control over a market, they have complete reign to implement many monopolistic practices. This is why no monopoly can do perfect price descrimination, but they can still do a lot.
The status of Microsoft's monopoly status shows up in the fact that Apple has to build in support for FAT, SMB/CIFS, and Active Directory into their computer in order to stay relevent. The fact that pretty much all peripheral memory devices come formatted in FAT is another pointer. The use of the MTP protocol yet another.
Microsoft has, on numerous occasions, been found guilty of anticompetitive and monopolistic practices.
Microsoft's monopoly on the desktop has been slowly eroding for some time, but it's still quite a powerful force. MS Office is probably a more powerful force for Microsoft right now.