The whole idea is reliability. My point is that Windoze by itself on an HD is very well behaved and I gets no more crashes. Well, maybe a lot fewer. On a one HD system it would inevitably come down every six to eight months (Bootmgr fails to load or HD is unreadable). Then I would loose all my docs etc. :(
Now when windows crashes My docs are all nice and safe on a separate HD. I just seems to me to be a better mousetrap this way.
The windows HD is a 120 gig with 90% free space (Ive done the partition thing and tried to be fancy using one partition for swap etc. but this time around I just formated and installed plain vanilla. The windows installer likes it that way...
Another plus is that only the Windows Hd ever needs to be degfragmented. The other HD is always un needed for dfrag. So defrag goes by fast. I do however, a lot of times have to run chkdsk because of file errors on one of the two HD every now and then.
RAID is okay for mirror and array 0 and although I am big fan of RAID from a long time ago, new HD's are very fast (Raptors and such) and getting faster. I prefer to mirror to another PC on network instead of onboard dual. But thats me.