Well, it’s open, so all (security) lacks/bugs can been solved very fast.
That also makes it secure.
It also runs smoothly on older PC’s.
So it’s also fast.
And, ofcourse, it’s FREE!!!!!!!!!!!!!!
So, enough reasons to switch from paid OS’s to free OS.
You’ve also got a bounch of choices.
You can even personalize your OS.
Also, the use and installation is easy.
Look!
Even more reasons!

The most recommended versions are:
Ubuntu.
Linux Mint.
OpenSUSE.
Fedora.
ManDriva.

And if you want your PC very fast, and don’t mind on the graphics, then choose (desktop enviorement):
IceWM.
LXDE.
FluxBox.

Now, let’s compare Windows and Linux.

Linux

Windows

Hardware

Support by Open-Source developers

Support by hardware makers

Software

Mainly open/free (and some closed/paid)

Mainly closed, paid and free

Weight

Depends on distro and DE

Very heavy

Price for OS**

€ 0-60

€ 199,99, € 309, and € 319

Market share*

0,94%

93,06%

Standards

Open standards

Own standards

Promoted by

Internet and human beings

Stores, TV, Ads, everything

Known motto

Share the freedom!

Developers, developers, developers, developers.

However you’ve seen how awesome Linux really is, some kind of people can’t use it.
Like PC Gamers, as an example, or professional Photoshoppers, Flashers, Dreamweavers, etc.
Or even students.
Their games or software don’t support Linux (and will most likely not happen).
So they can better keep using their Windows (or sometimes Macs).
Why?
Because people are way to lazy to port their work to Linux.
And sometimes, they don’t even allow that, for an unknown reason.

* As of Wednesday September 23th, 2009.
** Retail prices for, in the right order, Windows 7 Home Premium, Professional and Ultimate. As of Wednesday September 23th, 2009.