Depend on your OS, you can only setup RAID on 2000, Server and XP Pro, but you have to convert both harddrive to dynamic in adminstrator tools/computer management/disk management.
It's usually done via software, your motherboard should provide you with one. Connect the second harddrive (make sure they are identical, same size same brand) to the secondary sata connector and whatever software you will use follow the instruction on how to set up mirroring.
You don't have to reinstall your OS, it's just like adding a new hardware.
You can not go back to basic if you are in dynamic.