Assuming you have mysql and apache

1- backup everything
2- install linux
3- install mysql and apache httpd (probably php as well)
4- restore backups
5- adapt config from windows to linux (mostly paths and nothing more)

Be done with it!

Regards

This is very lengthy / tedious process. A lot of patience is required to do the job perfectly. Have u any spare system to test the same. It is better to do the entire job in a test pc and after getting success, u can implement the same in a production environment. Everything u have to overcome, suspecting only issue u may face hardware part. If u have AD (sure about that) then take extra care about that. Take all necessary backups not only in a single media / storage disk, take more than one also. Look here for more info: www.yolinux.com/TUTORIALS/LinuxTutorialMicrosoftWindowsNetworkIntegration.html

Good luck with the whole process!