We do this where I work with Vista and XP. Not sure how much applies to Server 2008. We basically have the extracted ISO image of the CDs on our server and we run a WinPE (WIM) bootable image on the USB drives with a batch file to map the share and start the setup process. We also have an unattended script so it doesn't prompt you for Product IDs or other stuff. The article linked above basically just turns the USB key into a bootable image of the original install DVD. The important part is where you format it properly via the steps in that doc. There are some other tools that will work but those steps in the article have worked for me and will work on any Vista box as far as I know.