If you installed Windows 7 after you installed Ubuntu, you probably did 1 of 2 things:
You either installed Windows 7 over Ubuntu and deleted everything, or when you installed Windows 7 in a separate partition you overwrote the Grub bootloader. If the first was the case, then you'll need to reinstall Ubuntu and Windows 7 on separate partitions. If the latter, you'll probably want to see this answer. Summarily, you can just boot into your Windows environment, go here and get the latest version of Grub4dos, and install it on the hard drive that you installed Windows 7 and Ubuntu on.