Its due to virus definately.
I reccomend to try Microsoft security essential Or you can try the below solutions if you donot want to change antivirus.
Click on the below link and download the file "AutorunExterminator"
http://en.kioskea.net/download/download-11613-autorun-exterminator
Extract it --> Double-click on "AutorunExterminator" --> Plug your flash drive now.
This will remove the autorun.inf files from your flash drive and also from drives.
step 2:
Click on "Start" -->Run --> type cmd and click on OK.
Here I assume your flash drive letter as G:
Enter this command.
attrib -h -r -s /s /d g:\*.*
You can copy the above command --> Right-click in the Command Prompt and
paste it.
Note : Don't forget to replace the letter g with your flash drive letter.
Now press "Enter".
Now check for your files in Flash Drive.
Step 3:
After that, download the Malwarebytes' Anti-Malware from the below link
http://en.kioskea.net/download/download-105-malwarebytes-anti-malware
Update it --> Perform "Full Scan"
Note : Default selected option is "Quick Scan".