LibreCAD is free, open-source 2D CAD. You can place the portable version (available via PortableApps.com) on a USB drive. It reads/writes DWG and DXF, uses minimal resources, and requires no license.
nanoCAD by Nanosoft offers a free version with core AutoCAD-like functionality. The installer is under 200 MB. While not officially portable, you can install it to a USB drive with some customization. portableautodeskautocad2010lite repack