As previously mentiond , start by cleaning up your windows, deleat any old files folders and other junk you no longer need, run your Antivirous and regestry cleaner [both in safe mode] re boot and run de-frag this can take all night depending on the size of disc and when it was last done, installing mint is simple, boot your installation media [pendrive/dvd] to run live , test that everything works ok [sound graphics, blue tooth, wi-fi, if your happy hit the install button when asked select install alongside windows and go, you will be sked about partitioning the drive [let it do it automatically] you will also be asked during install , if you wish to install non free/proprietary drivers say yes enter and follow the instructions, this is the method I have used on all the machines where people have kept windows and added Linux
Bwiz