Installing Windows 7 with RAID 5 Hardware omn DELL T5810

Hello guy as always i need your support...

we have a Dell Precision T5810 Arrived with Three disk(2 TB ) Configured with RAID 5. My Boss asked me to install Windws 7 but i have some Issue.

I'm installing Windows 7 with USB Stick, during the installation i need to install the driver to be able to read the disk. After Loading the drive i found 4 partition

Disk 1 Part 1 ESP 500M

Disk 1 Part 2 DIAGS 40GB

Disk 1 Part 3 OS 2 GB

Disk 1 Part 4 UNALLOCATED SPACE  3700 gb

but is impossibile to install WIndows becasue this error:

Windows cannot be installed to the this. The selected disk is of the GPT Partions.

How can i fix and install Windows? 

TANKS GIO