Linux makes a much more efficient use of resources available than Windows, wether it is RAM or CPU usage, windows can be a very fat system... (specially 7 & 8) For this reason Linux is a very good replacement for machines that are just enough to run XP today... (since XP ends its lifecycle in a couple of months)
In terms of security, Linux has several features and concepts built into its structure that make it safer than Windows. Since linux is designed with a multiuser UNIX structure, it requires root access to make any significant changes to the system, most users are limited in the scope of what they can break. Some distros also have context-based access to files and programs that prevent a rogue program from accessing files that are out of its scope (SELinux), the fact that it is open source also helps with its security since there are far more eyes looking at the code and finding bugs than you'll ever had in Windows...
And you can't forget the price tag... $0 vs a few $100...
Linux Mint is a great distro to start using Linux... it is a good idea to get comfortable with the terminal since it gives you a lot of power once you learn the ropes, and it is very necessary if you wish to move to a more solid distro later on (like anything from the redhat or slackware families)
I moved from Windows 7 about a year ago, first to Ubuntu & Mint later to Fedora... and I haven't looked back.
I m using Windows 8. It is one of the best OS.