If you have just reset your BIOS, or done a flash. You will need to go into BIOS and re-enable it BEFORE trying to start windows. With RAID 0, 5, or 0+1 it wont boot at all,
IMPORTANT!
If it is RAID 1 it will liklely break the array booting into Windows without resetting it, as logging into Windows writes a small amount to the drive, and then they would no longer be synchronized. Meaning your best bet is to reload the operating system.
HOWTO:
For Intel chipset boards:
At first boot, goto BIOS - Integratred peripherals - PATA/SATA Device and set Sata mode to RAID. Then press F10 to save and exit.
For NVidia chipset motherboards:
Goto BIOS - Integrated Peripherals - RAID Setup and enable RAID in the top option, then enable it for the ports that you have your RAID drives plugged into. Then Press F10 to save and exit.