RAID is a Redundant Array of Independent Disks. Depending on the computer, the hardware may or may not support this. If it does, it allows you to set up two or more disks at the hardware level - in the BIOS - that appears to the computer to be a single disk. Should a failure of one occur, you will continue to run normally but will receive a message that one disk has failed. Replacing it with the same size or larger disk ASAP will result in the new disk automatically rebuilding itself from the disk that hasn't failed. Then you're back to redundant disks. Its reliability control - insuring that the failure of a single component will not kill the whole system.
I recommend only using a RAID 1 for your PCs. That's two HDDs exactly the same size. Best they are also the same brand and model but try to get them with two different manufactured dates. Personally I like the Western Digital RED stripe drives. I'm having the best luck with them right now on my arrays and I'm convinced it's because they run a good 5 to 8 degrees cooler than an equivalent Seagate.
If the system does not support RAID then it is probably a mid to lower end PC. All high end PCs have the chip set that will support this.
I have even added RAID cards into computers that did not have the hardware support but I do not recommend this for the faint-hearted.
Now that being said and done, you can also do this through your operating system too. You however are going to have to have a Professional edition of Windows 7. Take a look at the links here and the videos and study them before beginning. And please please please do an IMAGE backup of your existing hard drive before you attempt this otherwise if something goes wrong you'll have absolutely nothing left.
https://www.google.com/search?q=windows ... gws_rd=ssl
A very practical way to do this is to simply buy two new disks to start with if you have the hardware capability. Create the RAID 1 with the new disks then clone your original on to the RAID. If you are doing it with Win7 pro as a software RAID then you can do the same thing but will have some fooling around to do to get your RAID to be your primary disk after you clone your original primary disk onto the new RAID then remove it.
If your computer is so old that it is still on Windows XP and running EIDE PATA drives and not SATA drives I would recommend you order a new computer preconfigured with a RAID 1.
So has Paul.
