This site will walk you through the whole process.
Make sure you have a large hard disk e.g 160Gb.Partition the HDD into 2 partitions.Install Windows 7 on the first hard disk and then install Ubuntu on the second hard disk. Now while installing ubuntu click on ‘Advance’ option and there select the second hard disk to load the grub boot loader. Now you can boot from any hard disk by selecting your any hard disk as the first hard disk from BIOS. Some motherboards have option to press a key which allows you to select the device from which you want to boot. You can use that too. But this is again unethical or not practical. So you decide what you want. Either you can do this way and need to be careful on every boot or you let GRUB takeover and be at peace of mind.Hope it works for you enjoy.
The most important thing you have to consider when creating a dual boot system using different operating system in a single computer unit is the partition. Your hard disk drive should have a partition of at least 2 partitions because two operating systems installed in a single partition like Drive C will not work at all.