Also, Defragging a hard disk regularly will shorten its lifespan immensely. Bad idea for critical info Toby!
As said, the best way to keep files safe is to have two copies, I personally would use a mirrored RAID array as said above, but that can be pricey. Alternatively, keep everything of value in one folder (My doc's?) and back it up very regularly.
But don't defrag frequently, the constant movement of the discs (rotating) and the head moving in and out to get the fragments and re-arrange them is a time consuming and extremely wearing process...
If you defrag weekly/daily that will shorten the lifespan and reliability of a drive greatly.
Differing opinions on this exist, but I'm definetely right
I never defrag my computer (mibbe once/twice a year, and only if I feel it has to happen). This kind of stuff is my speciality lol
Additionally, if you're using Vista or an NTFS filesystem (rather than a FAT variant) you're doing much more harm than good, as NTFS avoids fragmentation, as does Vista. Some fragmentation is inevitable and can't be avoided
Dave