You should look at various backup software then, I'd say, and maybe an external or networked hard drive.
Windows has it's own built in backup ability -- not great, but it's there. Norton has some. There is some freeware and shareware out there as well. I think you'll have more luck in finding a backup solution in case you ever need to do a clean install, than trying to turn a existing single drive system into a mirrored RAID 1 configuration. Also, most systems take a performance hit with some backup software (Norton Ghost for example) or some mirrored RAID 1 configurations, so if this is a game system, you may see some performance loss, depending on the solution you go with (occasional manual backups vs RAID or mirrored backup software solution).
Windows security arrangements makes backing up the OS a PITA much of the time, as well as trying to make copies of OS files; you may be better off backing up all your other data and updates (I tend to keep all updates for all programs in a folder on both my C or D drive and in my external hard drive backup folders). So long as you have the original OS CD/DVD, you can do a clean install of the OS and drivers if need be, then reinstall the backups as needed. It does take some time, so if what you have is mission critical, then yes, look to a good RAID solution, and be prepared to spend some money to do it right.
What I've found works best for me is two 70G hard drives in a RAID 0 configuration (data stripping) for the OS and some hardware intensive programs and games --- this is for performance, not backup ---- , while keeping my backups, files, music files, photos, and so forth in a 3rd drive on the tower, as well as copies in an external hard drive on the home network (which has folders for each tower and laptop as for backup and shared files).
For a true fault tolerant RAID 1 configuration, I think you will have to invest in two equal drives and look at a clean install of the OS and all files (after saving to another system/external drive/backup what you want to keep) to really do it right. Or you can go hawg wild with lots of hardware for RAID 3, 4, 5, or 6 configurations. An external hard drive and backup software will be a lot cheaper.
You can still use that second drive as a D drive and keep backups on it. If your C drive goes, then you replace the drive (if necessary), do a clean install of the OS to the C drive, then pull your backed up files from your D drive folders as needed. Just no way to access that D drive until the C drive is back up (which is why I have the networked external hard drive).
RAID configurations, especially 1, 3, 4, 5, and 6, are for fault tolerate, mission critical systems and sometimes performance gains. If this is the case, I think you'll need to invest a bit more to do it right, and you will be looking at a clean install, not trying to tack it onto an existing system. If you are just backing up a home/office computer, then backup software and an external (or networked) hard drive should work fine. External drives are especially useful if you have access to multiple computers, so if one is down, you can use another to access the files on the external drive.
