I have installed Windows 7 today and right after installed Ubuntu 12.04 next to it.
Now after rebooting the machine, it goes right into Win7 without asking me anything at the boot menu which OS to load.
In windows I can see that the ubuntu partitions are still there.
Thanks
As the GRUB is installed in usb disk not in your hard disk , I recommend you to boot the live usb, then install
boot-repair
tool and reinstall grub to your hard disk. :)Read carefully -> https://help.ubuntu.com/community/Boot-Repair