I have a machine with Windows on it, and I would like to be able to reboot to Linux. I am certain this is possible. How can I achieve this?
Which Operating System do you want to run?
You'll need to partition your hard disk for other OS. For details on how to do this, you can check out this guide.
I would advise trying out Linux, especially Ubuntu. Thus, let's assume that the OS you want is Ubuntu.
Two easy ways: the first method is to use Wubi Installer. Try this guide.
There is corresponding documentation on their websites.
This is very easy. Moden Linux distributions generally install a bootmanager, i.e. a piece of software that shows up on startup and lets you choose which operating system to boot.
It looks like this:
To get this, you simply download a Linux-distribution of your choice and install it according to the instructions. Along with the install a bootmanager will be installed and you are free to select the desired operating system on reboot.