Spybot Lite (portable)
You can make Spybot portable simply by installing it to your C drive and then copying the files to your USB drive. I've been tinkering with it lately, trying to shrink it's 11.5MB size on my USB drive. If you're focusing on detection and removal as the functions used on your USB version of Spybot, here's a list of files that can be deleted to reduce it's size:
1. All non-English license, help, and language files - This doesn't save much space, but it's better than nothing.
2. All files in the Updates folder except 'downloaded.ini' - It appears that Spybot retains the zipped update files even after applying those updates. I saved about 2MB by deleting them. 'downloaded.ini' keeps a record of the updates you've already downloaded, so deleting this will cause Spybot to think you don't have the updates you already have. 'online.ini' will regenerate when you check for updates. (The Languages, Plugins, and Skins folders will also regenerate when you check for updates, if you've deleted them.)