Check out "Puppy Linux" It can be loaded on a CD, and you can run it without changing ANYTHING on you computer. Most Linux distributions require installation on your machine, Puppy stays only on the CD, unless you decide to install it permanently.
The only thing you need to do, after downloading it and burning it to CD (or DVD), is open your bios at boot up, and change the boot order.
If you aren't familiar with that, the boot order just tells the machine wherefore look for the operating system.
If you change it to look on CD first, it will boot from the CD (if there is a bootable CD in the drive). Leave your hard drive as 2nd choice. This way, when you want to use Linux, put CD in drive, and reboot. If you want to run Windows, don't put it in the CD drive. If you leave another CD, or DVD in the drive, it will ignore it, and boot up off your hard drive (windows)
The huge benefit to Puppy Linux is that you don't need to commit to Linux, you can use it until you think you are comfortable with Linux, and want to make a permanent install. You will probably find it to be all you need. The larger distributions have the same Linux, they just include their own choice of other programs. If you need a program that isn't on the Puppy distribution, you can add it as needed.