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.)
3. TeaTimer.exe - Used by Resident TeaTimer, as part of a permanent installation of Spybot (can also be run seperate from Spybot). Monitors processes and detects known malicious ones that want to start and then kills them, then lets you create rules. TeaTimer also detects changes to IE settings. If you run TeaTimer from a USB drive while examining a system it might not detect any malicious processes simply because none tried to start while TeaTimer was running. Since it's only monitoring the system while running from the USB drive it might not detect a present malware infection on the system you're examining.
4. SDHelper.dll - Used by Resident SDHelper (aka Resident IE), as part of a permanent installation of Spybot. Blocks IE from downloading known malicious files. This DLL is also used by TeaTimer, however TeaTimer can run without it.
5. Blindman.exe - Used by permanent installations of Spybot to speed up the boot process when startups are disabled by Spybot. This doesn't save much space, but the file is pretty much useless for our purposes here.
6. Startup.tnfo - Provides additional info on startup programs. This info is taken straight from pacs-portal.co.uk (you can also find it at sysinfo.org).
Deleting all those files brings the size down to about 7.6MB and you still have all the functionality of scanning/detection/removal.
I'm trying to figure out what other files can be deleted too. Does anyone know exactly what each of the .sbs files in the Includes folder relate to? I know the .sbi files are used for scanning/detection, but I'm not sure about all of the .sbs files.