When you install Ubuntu Linux, I suggest that you consider either setting up a dual-boot system so that you may choose either Ubuntu Linux or Windows at boot time, or else use the entire hard drive for Ubuntu. (Using the entire hard drive for Ubuntu will wipe out Windows. You may actually wish to do this later after you start running Ubuntu Linux and find that you no longer need or perhaps even desire Windows.) To setup a dual-boot system, you may simply install it using its default installation mode (to install it side-by-side with your other operating system, such as Windows). I like to be cautious and allow for mistakes. So I recommend that before you install Ubuntu that you back up your important files on an external hard drive, USB flash drive, or on CDs. Then defragment your hard drive very completely. Even after the defragmentation program says that it is done, I suggest that you repeat it at least a couple more times to see if it will move the files closer together instead of allowing them to be spread over a wider section of the hard drive. (You might be interested to know that Ubuntu Linux defragments files on-the-fly so that defragmentation is not required with Linux.)
There are a number of methods that can be used for installation of Ubuntu Linux. (There is even a way to install it inside of Windows, but I highly do NOT recommend that. It would be dependent upon the Windows operating system in such case. You want to make your system amazing don't you?) The installation method I recommend is to place the "live CD" install disk (that you prepared earlier) with the Ubuntu Linux ISO file on it into your computer's CD/DVD drive. (Or you can place a flash disk, prepared as mentioned above, in one of your USB ports.) Power down the system and reboot. [Usually, a better and more thorough way to do this is to 1) Exit Windows and shutdown your computer, 2) Unplug your computer for at least 30 seconds, 3) Plug-in your computer and wait at least 60 seconds, and then finally 4) Reboot your computer.]
Note: If you wish to install Ubuntu from a USB drive/flash disk, follow the directions here. For some computers, the BIOS cannot be adjusted to boot from a USB drive.
Your computer should boot up on the CD if the BIOS on your computer's motherboard is set to boot via the CD/DVD drive before it looks to your hard drive. If it boots up into Windows instead of booting up on the CD, you will need to adjust your BIOS to cause your computer to look to your CD before your hard drive. To do this, you may want to follow the general directions given for the adjustment of the BIOS at the link for installation using a USB drive/flash disk, except that you will need to set the BIOS to boot from the CD drive rather than the flash disk. (You may need to reference your owner's manual or look on the Internet for your model of computer to see how to set the boot order or boot priority.)
When your computer boots from your Ubuntu Linux install disk, it may give you an option to "Try Ubuntu," which performs a temporary "install" without changing anything on your computer so that you can see how Ubuntu works before you install it for real. If you do this, a number of things may run very slowly for this demonstration, but at least you will be able to tell if the bit version of Ubuntu (32 bit or 64 bit) that you are trying to install will run on your computer. Sometimes the 32 bit variety of Ubuntu will either not install or else not run properly on computers with AMD 64 CPUs. (I suggest that you try both the 32 bit and the 64 bit versions, if your computer is compatible with each, to see if there is any difference in the two.) When you install Ubuntu Linux for real, just follow the prompts and it will give you the default installation, but first from the menu items available, check the CD for accuracy/integrity---if it gives you that choice. Near the beginning of the installation process, it may give you the choice for the system to be updated from the Internet while it is installing and/or to install third party software to enable it to play more sound and video, or for drivers (such for wireless devices).
For Advanced Users
If you are an advanced user and want to exercise more control over the process, I recommend that you choose the manual method to partition your hard drive and give Ubuntu a minimum of 7 Gb of hard disk space, but preferably at least 25 Gb (or more), and make sure that you indicate to format it with the advanced "ext4" file system. Be sure to give this partition a mounting point of "/". Also, make the separate Linux Swap part of the hard drive from 2 to 4 times the amount of RAM memory in your computer. No formatting is indicated for the Linux Swap section and it has no mounting point. (Note that during installation, the Linux Swap partition will automatically be formatted.) It is also best (for fast operation) to make sure that the Linux Swap section is physically either at the beginning or end of the hard drive, but preferably at the beginning of the hard drive. Because of this, you may prefer to first use the professional Parted Magic or else Parted Magic Clonezilla program (discussed below) for this purpose. (You run the Parted Magic or the Parted Magic Clonezilla from a bootable "live CD." You do not install it.) Then when you install Ubuntu, you may use the preprepared existing partitions for the installation. Click here for further discussion regarding Parted Magic or Parted Magic Clonezilla. Also, if you have two hard drives (or an external hard drive), you can have the GRUB to be installed on the hard drive that does not have Windows on it, and then set the BIOS to boot from that drive. In that way, the drive with Windows on it is unaltered and the MBR is not overwritten. You will still be able to boot into either Ubuntu or Windows from the GRUB menu. If you use the option of installing Ubuntu on an external hard drive, there will be a performance decrease because the data transfer speed will typically be slower.
General Consideration about External Bootable Media
It should be noted that some computers may not boot up properly when some external hard drives (or perhaps certain other external devices that the computer may view to be possibly bootable) are plugged into them before they are booted. If you wish to boot from an external hard drive, do some research on that particular drive and find out if anyone has had problems with using it in that fashion.