You basically have 3 options for installing Ubuntu linux.
1. Wubi. This installs ubuntu linux so that it appears to be running on windows and can be removed with the window add remove utility. This is not the ideal way to install ubuntu and if windows becomes corrupted you will also lose ubuntu.
2. Install ubuntu in a virtual machine. You can install virtualbox in windows and then install ubuntu in virtualbox. You will have to dedicate some computer resources to the virtual computer (cpu cycles and ram). Since you will be reserving some computer resources for virtualbox you might notice a slow down in your computer depending on your computer specs. In order to run ubuntu you first boot into windows and then open virtualbox and start ubuntu. You do not have to reboot to switch back and forth.
3. Dual both both OSes. You format separate partitions for ubuntu and install ubuntu on the new partitions. When you boot the computer you select which OS to run and it has the full resources of the computer. You cannot press a button to switch back and forth but must reboot.
Each method has it pros and cons. Depending on your use for linux, your computer resources, and your backup methods which method you select will differ.
Since linux is different from windows it is important to read the linux documentation and understand how linux works. It is possible to overwrite you windows install if you make a mistake during the dual boot install.
You can take an on line tour of ubuntu to get a feel for it. Wehn you burn the ubuntu iso to a cd you can run ubuntu from the live cd (or live usb flash drive) to run it without installing it. It will run a little slowly because it is not using the hard drive. Be sure to read the "Show Me How" parts on the ubuntu website. Also check the Desktop Guide for which ever version of ubuntu you try.
Ubuntu is not the only linux distro or even the best, but it is a good place to start because it has very good instructions and has all the necessary drivers available, and is easy to get running. If you decide to stay with linux there are hundreds of other distros that you could try depending on your use for linux, knowledge, and desire to have more control and get into the fine details of customizing and reconfiguring it.
Linux cannot run windows programs natively but some windows programs can be run with more or less success under wine ( a windows sort of emulator). Linux does have its own programs that will accomplish the same tasks as windows programs (except games).
have fun