If your USB disk is affected by Virus, your PC also may be affected easily though you are using an Antivirus. There is a tool named "USB Disk Security" is very useful for protecting your PC against viruses spread through USB Device. Install the tool along with antivirus. This will work against "autorun.inf" virus which makes pen drive look like a folder. For download: http://www.zbshareware.com/download.html