Ubuntu Linux is an extremely capable and secure operating system. In addition, it has a reputation of being extremely stable and fast. Ubuntu Linux is very user-friendly, and many people prefer it greatly over Windows. You may also notice a speed increase in your computer. If you have an older computer, you may especially be surprised. Before you decide to install it, read about it here and here. It is also offered pre-installed on some new computers. Some people decide to not even use Windows any longer after they experience Ubuntu Linux and see what it can do. Example link. You can also run Windows XP, Vista, and Windows 7 inside Ubuntu Linux (through a free program called VirtualBox), if you desire.
To install Ubuntu Linux, you must first download the appropriate install file (an ISO file). If you wish to install Ubuntu for either a desktop or laptop/notebook computer, you can download it from here. Be sure to click on the 64 bit option if you wish to install the 64 bit version, which is to your advantage if your computer has more than 4 gigabytes of RAM memory. However, there is something to be said about flexibility also within the 32 bit version, because a few more applications and programs are made for 32 bit Linux. For example, if you want to run RealPlayer, at this time only the 32 bit version is made for Linux. On some computers (for example, some that use the AMD 64 bit CPUs), the 32 bit version of Ubuntu will not install. If it doesn't install, it is likely that the 64 bit version will. The 64 bit version may theoretically give slightly greater speed in some cases, such as when running software made to take advantage of newer 64 bit architecture. Many of the newer computers (regardless of whether they use Intel or AMD CPUs) will be compatible with either the 32 bit or 64 bit versions of Ubuntu. Ubuntu Linux also handles memory well in that it will run on as little as 256 Mb RAM, but it is best if your computer has at least 512 Mb RAM.
Compatibility with older computers: Please note that Ubuntu Linux version 10.04.1 LTS (which will be supported until sometime in 2013) is the last version of Ubuntu specifically designed to support PC computer CPUs prior to the i686. Some people may use it to make their older computers "sing."
Also, at this link are directions on an alternate method of installing Ubuntu Linux from a USB drive/flash disk (such as when installing to a netbook).
You may be able to also use this alternate method for installing Ubuntu Linux on either your notebook computer or your desktop, if the BIOS on the motherboard in your desktop is configured to "look" (at boot-up time) to your USB drives or USB flash disk before looking to your hard drive. See the directions here. For some computers, the BIOS cannot be adjusted to boot from a USB drive.