This is a great article: Using Windows XP to Make RAID 5 Happen
Here is the important part: Windows XP has the ability to create/handle Mirrored, RAID 5 volumes, but it is “disabled”. There is a way to “enable” these features. This hack is described in the mentioned article.

In case it’s not available there:
you need to modify these 3 files:
C:\windows\system32\drivers\dmboot.sys

Before:
0000f1f0h: 74 00 54 00 79 00 70 00 65 00 00 00 57 49 4E 4E ; t.T.y.p.e…WINN
0000f200h: 54 00 00 00 53 45 52 56 45 52 4E 54 00 00 00 00 ; T…SERVERNT….
After:
0000f1f0h: 74 00 54 00 79 00 70 00 65 00 00 00 53 45 52 56 ; t.T.y.p.e…SERV
0000f200h: 45 52 4E 54 57 49 4E 4E 54 00 00 00 00 00 00 00 ; ERNTWINNT…..

C:\windows\system32\dmconfig.dll

Before
00005140h: 4C 41 4E 4D 41 4E 4E 54 00 00 00 00 53 45 52 56 ; LANMANNT….SERV
00005150h: 45 52 4E 54 00 00 00 00 57 49 4E 4E 54 00 00 00 ; ERNT….WINNT…
After
00005140h: 4C 41 4E 4D 41 4E 4E 54 00 00 00 00 57 49 4E 4E ; LANMANNT….WINN
00005150h: 54 00 00 00 00 00 00 00 53 45 52 56 45 52 4E 54 ; T…….SERVERNT

C:\windows\system32\Dmadmin.exe

Before
00001c30h: 73 65 72 76 65 72 6E 74 00 00 00 00 6C 61 6E 6D ; servernt….lanm
00001c40h: 61 6E 6E 74 00 00 00 00 50 72 6F 64 75 63 74 54 ; annt….ProductT
After
00001c30h: 77 69 6E 6E 74 00 00 00 00 00 00 00 6C 61 6E 6D ; winnt…….lanm
00001c40h: 61 6E 6E 74 00 00 00 00 50 72 6F 64 75 63 74 54 ; annt….ProductT

and copy a copy to C:\windows\system32\dllcache
But don’t forget about windows file protection, do this in safe mode, or form boot cd/usb…

Windows service pack 3 overwrites these files (2 of them anyway). So if you did this on SP2 and updated to SP3 after that you won’t be able to access mirrors, raid 5… The good news is this works on SP3 too, so all you need to do is reapply this “patch”. (A new windows update, or “security patch” might disable this feature in the future. But I think microsoft is concentrating on Vista these days)

Why is this good/useful anyway?
First of all, do you want to keep your data safe?
Let’s be clear, this alone won’t keep your data safe, this is just one step. You need external backup (possibly a location far away) too.
There is several reason for and against hardware raid. My top reasons against HW and for SW RAID is: HW change/compatibility/portability, completely custom raid volume sizes/disk sizes. (but RAID 5 is slow on SW raid)
Well if you do not have windows 2003, but you want to keep your data on SW RAID, this might be your solution.
Personally I do have a win2k3, but only one. And if it’s not working for some reason I want to be able to get to my data fast from a second computer (win XP running on it). I don’t use raid on my backup xp, but I might need it someday…