I'm surprised no one has stated "dual boot", which means having two OS installed on one computer. A Google search on dual boot windows 7 and linux will yield many references. Installing a Linux OS on your computer while leaving Windows 7 will allow you to choose which OS to boot when you start your computer.
Another option is to install a virtual machine program such as VMware Player or virtualbox and then install the Linux OS as a "virtual machine", or VM. With this type of configuration for running a Linux OS, you computer starts with Windows 7 as usual and then you can start the VM program and start the Linux OS you have installed as a VM. In other words, you run both Windows, the host OS, and Linux, the guest OS, simultaneously.
Still another option, at least with Ubuntu and maybe other Linux OS, is to run Ubuntu as a program in Windows. To do this, use the
http://www.ubuntu.com/download/ubuntu/windows-installer