There is actually a big advantage to having your OS on a different disk. A few, really. Your data drive won't be affected by all the disk activity caused by normal OS functions such as swapfile seeks and the like. Also if the OS needs to be rebuilt it will be much easier as far as connecting to the array. The disks in the array will have information about the array config on them, so the OS can see this and give you the option to reactivate the array. One problem though now that I think of it: I'm not sure XP can do raid 5, I think only 0 and 1. Matter of fact I'm certain this is true.
