There is a Windows Command Line Scanner (for Windows), however, this is not a free product,.This product comes with daily signature updates.
If you require any more information on purchase, please let me know so I can direct you to the appropriate people.
There is a way to have a portable command line VirusScan for free but cannot run automatic auto updates, you can download the superdat file and extract to the USB with the /e switch the engine and the dats files, once you have these files you can run scan.exe from prompt.
If you want to update just download the datxxxx.zip file and extract on to the scan folder on your USB.
This practice can help to scan infected machines.
If you really want a portable AV running on a USB stick I recommend The McAfee Encrypted USBs I has an older biometric Phantom from Safeboot with the new firmware installed and I m very satisfied.
Please note that all downloads are subject to licensing and this includes the DAT and SDAT files as well as any engine used.
NOTE: As of 1 April 2010, scan.exe will no longer be included as an executable in SDAT files.
See KnowledgeBase article KB68671 (available later today).