If you have an older computer that is getting slower and slower with every upgrade/update of Windows or OSX then you may want to consider another OS, Ubuntu. Ubuntu is a free and open source software that is probably the most popular Linux distribution for desktop computers.
The Ubuntu Desktop version is used for personal computing purposes and has a graphical user interface that is similiar to the one used in Microsoft Windows. A computer that is now running Windows XP, Vista, Windows 7 or even x86 OSX will be able to handle Ubuntu. The minimum hardware requirements are: 700 MHz processor (Intel Celeron or better); 512 MB RAM (system memory); 5 GB of hard-drive space (or USB stick, memory card or external drive but see LiveCD for an alternative approach); VGA capable of 1024×768 screen resolution; either a CD/DVD drive or a USB port for the installer media and Internet access.
The download is available at the Ubuntu.com website. You can try the OS without actually installing it by running it from a bootable DVD or USB stick. Once you have the ISO on the DVD or stick you simply put it into the computer and restart it. At the welcome screen you then choose Try Ubuntu. If you are ready to take the plunge then you can choose to install it along side your existing OS and have the ability to choose the OS that you want when your computer is booted.
On most computers Ubuntu will be faster than the OS that is already existing on your computer. It comes with a number of built-in software applications, including: web browsing (Firefox), office applications (LibreOffice Writer, LibreOffice Calc, LibreOffice Impress), email (Mozilla Thunderbird), music (Rythmbox Music Player), photos (Image Viewer) and video (Movie Player). The LibreOffice Productivity Suite also has a Windows version which would allow you to edit your Ubuntu files on any Windows computer. The Ubuntu Software Centre gives you access to thousands of other applications, that you can easily search for and install directly from the Dashboard. Also available is Ubuntu One, your personal cloud service that gives you 5 GB free.