Apple said that they were the first to get rid of floppy drives, and they were the first to remove the DVD drive on their Macbook Air. If all laptops, notebooks, and netbooks are DVD drive less in the future, USB is the only interface of recovering or installing your operating system.
I own a Sony VAIO-P (VGN-P15G) and the only way to re-install my OS is by having a external DVD drive. There is a tool from Microsoft called Windows 7 USB/DVD Download tool that allows you to make a USB thumbdrive to be bootable exactly as the Windows 7 RTM (Release to Manufacture) DVD. You would have to acquire a Windows 7 DVD, Any Windows 7 RTM DVD works. They're all the same as of this day. The only difference is when you enter your original serial key for activation.
Use any free ISO maker you can download. I use Ultra ISO. Other ISO should work fine as well but i haven't tested it. Go figure. Make the ISO from your Windows 7 RTM DVD. Once that is done download Windows 7 USB/DVD Download tool select the ISO and choose USB thumbdrive option and let it burn...
Want to use Windows 7 Ultimate for free? There is a built in feature in Windows 7 that allows you to extend your trial of Windows 7 from 30 days to 120 days extra before activation. If you don't have the money to buy the original software, just use the trial period to the extend it. This process is called "re-arm". Is something like re-arming a bomb timer. Follow the instruction in screenshot below:
Note: You should only re-arm when your trial period of 30 days is over. If you re-arm on the 15th day trial then it would go back to 30 days and you loose one re-arm cycle. When you reached your re-arm limit, reformat your computer and re install Windows 7 from your ready made USB Thumbdrive. You do it once every few months. Thats about time to spring clean your pc anyway :) thats what i'm doing now.