I have installed Windows 7 on lots of machines now and most of them (like my Samsung netbook) don’t have DVD drive. So they way I do it is I have a 4GB USB memory stick and copy the Windows 7 ISO to the drive. Then on the machine I want to install Windows 7 on I downloaded and install Virtual Clone Drive, a free application that enables you to open ISO files.  Once you have installed Virtual Clone Drive you can double click on the ISO file and the Windows 7 will run. You can choose to upgrade your existing install or have a clean install of Windows, if you do that without formatting the drive it will create a Windows.Old file with the old Windows install in it which is handy if you need to get any drivers from the old Windows install