A complete operating system for your PC for £0. Much cheaper than Microsoft Windows 7 plus more features than 7, all free. Just download and burn to CD! You can even try before you install without making any changes to your PC! Arguably the best Linux distribution currently available. It is as easy to install as a Microsoft OS, for most PC's it will recognise all your hardware on installation. In the unlikely event something does not work there is a large user base and Google usually comes up trumps with an easy to understand solution. A good, lean, secure, and virus-free alternative to Windows! Ideal for Netbooks and older spec PC's that may struggle with Windows.
Comes with OpenOffice, a viable alternative to Microsoft Office for most users and able to open and save to Microsoft's formats. Also features a searchable Software centre with lots of free software for a variety of purposes. For almost anything you want to do, or could do with a Windows PC, the software is there! A good range of games is also available. And if you miss some of your windows apps there's even WINE, a free WINdows Emulator - but if you're not willing to make the jump straight over to Linux just yet, you can install it next to Windows and choose which loads each time the PC starts.