KitGuru.net posted a guide about installing Windows 7 from an USB Drive

One of the best ways around this is to install Windows 7 from a USB drive, which can cut the install time down to around 15 minutes (from 30-40 via DVD) if you use a fast drive. I am constantly being asked to explain to friends and colleagues the best way to do this as many of the guides online are too complex, or require third party software which doesn’t always work. With this KitGuru guide all you need to ensure is that your motherboard supports USB booting … which is pretty much a common feature now.

  How to Install Windows 7 from an USB drive – the easy way