I've seen many try to set up a RAID 1 array without re-installing the OS. It can be done with some RAID controllers. I.E. I have 2 onboard RAID controllers. My Silicon Image RAID Controller has to the option to 'create with data copy' which means that the controller would've just copied data from the source drive to the mirror drive to create the array. But my NVIDIA RAID Controller doesn't give me this option.
Unfortunately not all controllers give you the same options, even though they give the same array options. Many usually create the array during an OS install.
Make sure you enable the SATA drives in the BIOS and enable RAID, if applicable.
The safest way you can set up RAID 1 is to save all your important files onto another medium then go for a complete re-install.
Before you re-install you need to create a RAID driver on a floppy disk. The RAID driver should be on the Driver CD that came with the mother board. It won't work without this. Once you have the floppy disk with the RAID driver on it you can go ahead with a complete re-install.
During the OS install watch at the bottom of the screen. It will give you the option to install RAID driver by pressing F6. Do this and it will prompt you to insert the floppy disk. Follow the instructions. When done it will carry on installing Windows. You then select the array to load windows on to after which it will format the array and continue to install.
The above process is the best way to create the array due to the fact that the OS is on it.