Which OS should be installed first for dual boot? Windows 7 or Linux Mint 9?

If you want the proper dual boot, do windows first. If you do Linux first, the Windows boot loader will overwrite grub, and NOT detect your Linux install (on purpose... really), so you will have to go in and do grub again to get your Linux install working. If you install Min9 on a disk that already has windows, Mint will detect it and put in a reference in Grub for you.

Thats for a proper dual boot system.

In my humble, dual booting is no longer the way to go if you want both operating systems. Since you can get a really solid hypervisor for free these days, I prefer doing a native Windows install and then running Linux in a VM. Linux will install like cake in a VM, and then you can access both system at the same time if you need to. You can even transfer files between them on the fly!

But yeah.. if you want to do a dual boot. Do Windows first.

You can install win 7 after linux but then you will have to take the extra step of reinstalling grub (linux bootloader) after the win 7 install.

Windows first, Mint 9 second is working for me.

my only experience is with windows XP and Ubuntu.

sequence which worked was windows before Ubuntu.