Huge choice of distros to choose from.
Most distros are free.
Most distros provide a Live CD so that you can test it before installing.
Most Linux software is free.
Viruses, spyware, malware etc are not an issue.
Stability!
Extremely customisable.
Disadvantages
Your choice of games is greatly reduced.
Steep learning curve.
Not all hardware is supported (check before install)
There's probably more of each but I can't think right now.
Oh another advantage of running Linux, is that you are learning how to use it. Which is a good skill to have to put on your resume if you have any sort of computer related job.