You need to add the Mandriva boot instruction into the windows 7 boot section as an option.
Check under windows 7 start up options.
I am however uncertain what the entry should be in the windows 7 bootloader to launch mandriva.
I would have thought it better to install Mandriva first and then add windows 7 leaving the GRUB bootloader manage the booting of the various OS.
On a separate matter: RAID 0 provides no safeguard to drive failures which will lead to total data loss.
