How to choose primary hard drive

[ubisoftblackbox]

i have 2 sata hard drives

320 gb

1000gb

320 gb has windows 7 installed

1000gb has windows xp installed

when i turn on my computer windows xp starts loading.and i can also access the partitions of the other drive when i click my computer.question is how can i make windows 7 load?or how can i make the 320gb hdd primary?
motherboard:msi 760gm-p33

 

[ubisoftblackbox]

got my answer