That wouldn't work, at all. Like I said, you might want to consider installing windows to its own 250/500GB drive, and image that drive to the RAID5 array. I dislike the OS it self being in RAID unless the array is 100% proven to work with out it failing entirely at random. Especially if you are using a single install only version of windows. (That ultra cheap OEM you picked out can only be installed and activated once in its entire life)
If you REALLY desire a multi-partition raid5, use the raid controller to create the separate partitions. Just remember, if you loose enough drives, you loose ALL your partitions.