Malware often spreads itself by using the Windows autorun feature to install itself on USB drives. USB Immunizer prevents that from happening by placing a special, non-removable version of the autorun.inf file on any USB drives or SD cards you choose, meaning they're not vulnerable to this form of attack.
Developed by security software vendor BitDefender, the software is pretty simple to use: just launch it, plug in a drive, and choose the 'Immunize' option. You can also use the package to disable autorun entirely when removable media are plugged into your PC. Windows 7 itself blocks autorun from working with removable media, so this is likely to be more useful on older systems.
USB Immunizer is a free download for Windows. (When you close the program, it asks you to sign up for a security newsletter, but you can ignore that request.)