This is a simple way to unhide the folders in a pen drive which have become hidden after virus infection. I too have tried this and works.
1.start->Run->cmd
2.Then type the following
attrib -h -r -s /s /d h:\*.*
where h:\ is the drive letter of the pen drive.
3. Wait for few minutes and then check ur pen drive, the folders will be unhidden.
~~~~ERG WIFI~~~
