Linux is free.
Linux is gratis.
Linux is stable.
No virusses.
No serial numbers.
No malicious software (malware).
There are companies that do not allow the use of Windows for the above reasons, see The Register.
Good language support.
Boot from the Ubuntu or Debian Live USB stick (or DVD) and check that everything works before installing.
Easy installation in just a few steps, especially with Ubuntu and Debian.
Linux also runs on older hardware, no need to (always) buy a new computer.
Performing updates is system-wide; the entire operating system and all software.
Install software with one click, from over 58,000 packages, something for everyone.
Lots, lots of help on the Internet, for example via Ask Ubuntu.
Various desktop environments available such as GNOME in Ubuntu, KDE in Kubuntu, Xfce in Xubuntu, MATE in Ubuntu MATE, or LXDE in Lubuntu.
You can work as usual with Office (LibreOffice), IM (instant Messaging) (Pidgin), Web (Firefox), and Mail (Thunderbird), see also Open Source for Windows and Open Source for Mac.