RAID names a concept of using several hard disks as if they were single. It's the abbreviation for Redundant Array of Independent Disks (I guess and wont verify this now). There are 7 or 8 different RAID configuration options, but the most common for desktop PCs are NRAID (JBOD), 0 and 1. This might be because most modern mainboards only support these three. Perhaps they only those 3 modes because that's most common for desktops... however.
NRAID is actualiy no RAID, but it's JBOD (just a bunch of disks). This is because they aren't redundant, but they are just glued one behind the other. Well the glue is of course virtual: both disks are handled as one larger disk. E.g. if you have two 100 GB hard disks, you could make them act as a single 200 GB hard disk. That's smart, isn't it? Well no, not at all. Hard disks usually store sensitive information and should be very reliable. In fact by using JBOD you reduce reliability to the half.
RAID 0 is a performance gain: two hard disks are used as if they were one again and they will respond up to twice as fast as a single disk and have the capacity of both disks together. Now, is this smart? No no no, as the rather small performance gain for you desktop will be paid with 50% of reliability again. So if JBOD and RAID 0 are no good for you, lets see how useful the last applicable desktop RAID is.
RAID 1 is a reliability gain: two hard disks are used as if they were one again. It's recommended to have two disks of the same size, because otherwise you'll reduce size of the hard disks to that of the smaller one of both disks. Both RAID 1 disks will contain a complete set of data. If one fails, you can simply attach another disk to the computer's RAID controller and won't have any data loss. This is very nice in theory. But what if you ever want to port the disks to another computer, that maybe just doesn't have a RAID controller or doesn't support that specific RAID format? No no no and again: no. RAID is not made for desktop PCs. And it is not made for Windows.
If you want to have your desktop files and operating system save, no expert nor geek would recommend RAID. Playing arround with RAID is fine. But there has already been too much data loss due to RAID. A reliable desktop requires a real backup concept. Best practice therefore is to install a backup software and frequently store backups on DVDs or additional (preferably external) hard disks.