Well, the Avira scan found 4 infections on my machine. First time in... maybe 10 years or so, and I know how it got there. Flash drive.
Here's a tip I learned a few years back: On any USB drive, make a folder called, "autorun.inf". Not a file, a FOLDER.
Most viruses that spread via USB drives will put a text file called autorun.inf on your drive, along with a hidden program. An autorun.inf file is a standard file that gives instructions to Windows to run a program. Pop it in your computer, and it runs and you are infected.
But with a folder there by the same name, the virus can't write the text file, and the virus doesn't run. Cheap, free, and probably 90% effective, if not more.