You don't need to remove RAID to do what you want to do. You just have to create two partitions. Go inside Disk Manager (Vista: Control Panel>System & Management>Administrative Tools>Computer Manager>Disk Manager) to resize the primary partition ( C: ) and create a new one of the desired space in the unused space. It will work exactly as if you would have 2 disks ( C: and D: ) and you keep the increased speed of RAID 0. If you are at a loss on how to do it with the little steps I've given you can google "partitioning the hard disk" or "create two partitions in a single disk"; you will find better explanations.
However if you still don't want to do RAID you have to disable it in the BIOS. Usually you find the option in the menu where the SATA drives are listed. Usually you have to press "ENTER" on the drive you want to change and you change it from RAID to LEGACY IDE. Do the same with both drives and no more RAID. You have to reformat everything however, take this in mind, you willl lose ALL data.
As I said much better to just create two partitions. No need to lose all data and you can do it in the OS and you still have the increased speed of RAID 0.