A very handy tool is using the USB as a cleaning kit with programs and applications for saving infected computers. This is useful when the computer is disconnected from the internet and cannot download the software required to clean the computer. It's also a good kit when you're helping a friend rescue their virus infected PC.
A good revival kit will be packed with a range of applications and programs from the following categories:
Spyware Scanner: The more spyware scanners, the better. No single utility finds everything. Some good ones are Ad-Aware (free), Spybot Search & Destroy (free), and PestPatrol (free trial). None of these will run directly from the USB, so put the installer files on USB for a quick installation on the infected PC.
Antivirus Tool: A good antivirus tool that runs directly off the USB is Avira AntiVir (free edition). After removing the infection to make sure your PC remains protected use: AVG (free edition), Avira AntiVir (free edition), these have free trials: Norton and Titanium have 30 day free trials.
Disk Wiper: When worse comes to worst, and the infected PC cannot be saved, a secure disk shredder is helpful to have. A number of disk wiping programs run from the USB including: Kill Disk (free), and Eraser.
When using the security programs be sure to keep them updated, otherwise they may not be much help when fighting against viruses and spyware.
Hope this helps