- Always scan your USB drive.
-Sometimes your friend comes to your room and asked for your computer for casual stuffs - surfing, finishing her work or sort of that. Make sure he scan her USB drive with an antivirus. You can easily find one, which is free, for example "Microsoft Security Essential" or "MSE"
- Do not download a suspicious attachment
-Somehow you might get an email from unknown people. If you happened to download the file, make sure you scan the file first - in order to reassure it is clean and pristine
- Use Linux
-Well, Windows virus is not compatible with Linux. Different architecture for real. This, for the case of normal executable file. Application based attack will not being discussed here. Linux is free, and I have been using it for almost 10 years without any virus infection.
Lastly, stay safe!