Microsoft has enabled a lot of great features with the maintenance side of Windows 7, including the installation of Windows 7. There are guides coming out the wazoo on how to create a Windows 7 bootable USB Drive and a Windows 7 Restore CD, but we want to create a Windows 7 Restore USB... right? You might also be asking "Well, if I can create the bootable USB install and it already has the repair abilities, why do I need this?" My response is: If you aren't installing windows, why do you need all of the installation files? It's the difference between 200MB and 3GB...
I am going to assume that you already have a 32-bit or 64-bit copy of windows 7 and that you know how to follow the many instructions on how to create a full blown bootable Windows 7 USB Install. What I will provide are the specific files you will need to create a bootable Windows 7 Restore USB.
Basically, just follow the instructions on Intowindows on how to setup the USB drive to prep for copying the files. Follow the steps on that page down to step 8 and STOP. (Do not follow step 8 or 9!)
The specific files you will need to copy over to your USB drive to ONLY make it a restore drive, are the following: ( "\" being the root of your Windows 7 Disc)
\bootmgr \boot\bcd \boot\boot.sdi \boot\bootfix.bin \sources\boot.wim
If you don't have a Windows 7 Disc, or you need a 64-bit, but don't have it, check out the guys over at NeoSmart to pick up a copy of the .iso for both 32-bit and 64-bit rescue disc's. Then, use 7-zip or a similar program to extract the files from the .iso to copy over to your USB drive.
Keep in mind, this is NOT a DOS alternative, even though you will have access to a DOS prompt... It will not allow you to run regular DOS based programs... to do that, I recommend following the guide at Perley's Pontificating Place