Most people use Windows for a computer Operating System, and most of them do so because they are not aware of some much better alternatives, Mac OS-X and Linux.

Why Mac is better:

No Viruses:

To this day, Mac OS-X has completely no viruses (only a couple of Trojans which require you to give them your password first).
Windows fans usually say it's because it's not popular enough but that's not entirely true, for instance Windows has millions of viruses and Mac OS-X has about 2% market share so you'd say Mac OS-X had about 2% the number of viruses Windows has.
The truth is, it doesn't, it has no viruses at all.

It doesn't crash as often:
Most Windows users think that a computer that crashes software or display a blue screen is their fault (my parents always told me that I clicked too fast...).
This is completely untrue, Windows has always been very unstable and software crashes all the time (try installing Windows XP again without service packs to see what I mean).
Under Mac OS-X software also crashes, but not as often, Windows users complained about Adobe Flash CS4, I worked with it for a year on Mac and had no issues.

Your computer will remain fast:
If a Windows computer is a couple of years old it get's slower, much slower.
Most people will tell you that it's not true, it just feels slower but it is actually true.
Your drive is being fragmented all the time and it slows down your computer immensely, you can defragment your Windows with the built in tool, but it's slow and very poor.
In Mac OS-X, most files are defragmented automatically so you don't have to worry about this.

Free as in Freedom:
Windows is a completely closed system, only Microsoft can edit it.
Mac OS-X' kernel is completely open source, allowing people to alter it to suite their needs or find security leaks and submit them to Apple.

Ease of use:
The first time you'll use a new Operating System it's always hard, but learning to use Mac OS-X is very easy, you'll figure it out in a day.
Everything is build from the ground up to be easy but you still can do anything with it.

For instance, when installing a new version of Windows, you'll have to delete all your files from your hard drive (not in Windows 7 anymore, but it still goes if you're switching languages).
In Mac OS-X, you buy a new Operating System (which are released more frequently than Windows but they're much cheaper) you put it in your disk drive and press install, return to your computer 30 minutes later and it's installed with all your files and applications still on it.

the bad:

-Not much software yet:
Of course Windows has much more software, but this is because it has much more users.
For me Mac OS-X has everything I want, I have my Adobe software and I have some games.
A lot of software is also available for Mac OS-X or has at least an alternative, ask the supplier if you're not sure.
But, Mac OS-X is growing and so is the number of software it runs.

-Only runs on Mac hardware:
This is also a good thing, because it makes sure you'll never have a corrupted driver for your standard hardware and the Operating System is completely optimized but it also means you have to purchase very expensive hardware.

If you can spend the money, and all the software you need is also available for OS-X then buy a Mac.
For the people who need Windows software, there are multiple solutions that allow you to run Windows software on a Mac like Wine (also available for Linux) or even install Windows next to your Mac OS-X, this is supported and encouraged by both Apple and Microsoft.

Why Linux is better:

No viruses:
Hey, familiar? Linux and Mac OS-X are both based of the ancient (about 50 years old) Unix system for managing accounts, it ensures you can't do any system tasks without first entering your password so viruses have to ask you your password before anything happens (Windows tried a system like this too with UAC but that was more annoying for the end user than for the viruses).

Makes old computers behave like they're new:
There are many different Linux distributions (versions of Linux made by different people and companies), a lot of them are nearly equal to Windows in terms of use like Ubuntu, but there are also Linux distributions that are designed to run very fast on old hardware.
For instance, a very old laptop which ran Windows 98 can not run Windows XP, and thus can not run much new software.
Try DSL (Damn Small Linux) or Puppy Linux on this same computer and it will run all the newest software that Linux has to offer.

Great way of installing software:
Remember looking for a piece of software you couldn't find? Many Linux distributions include a package manager for installing software, so you just have to search for the software in a very long list and you can download it legally and for free.

Will not crash as often:
Linux is the most stable Kernel of the moment, you will hardly have any crashes whilst using it.

Free as in free beer:
Most Linux distributions (including the best) are completely free of charge, remember Windows? Windows costs $30 for the basic version that doesn't allow you to do anything, and $100 for the normal version that still doesn't have all features.

Free as in freedom:
Say Microsoft will stop developing Windows tomorrow, what will happen?
Windows will not be updated anymore and become virus invested to the core, and you won't be able to do anything about it.
If development for Linux stops you can edit the source code yourself, or other people will!

The bad:

-Not much software yet:
Linux does not have as much software as Windows either, but they do have a lot of free counterparts for paid software.
We also see many developers making their software available for Linux too nowadays, it get's more every day.
-Quite hard to use:
Linux can be very easy for some people who don't do anything but chat with their friends on MSN or browse the internet to go on Facebook but for people who want to delve deeper into the Operating System it can be quite hard.
Your hardware might also not be supported which can be very annoying but if you have a popular piece of hardware it's usually supported.

You should only stick to Windows if there's really no substitute for the software you use but you can also install Linux whilst keeping Windows.