|You don't have any antivirus ?|
First thing :
If you don't have and you don't want to spend money than use free versions
I would recommend Avast 7 free antivirus and Malwarebytes free anti malware .
Scan Your Drive with these programs and remove the viruses and yes keep that in mind if your USB also contains any other viruses and you have .exe files on your usb than forgot about recovering cause antivirus and antimalware will remove your programs with virus on your USB but if you have only multimedia files like mp3, avi etc be tension free than :D