4. Autorun.inf virus :

Cause :
u cannot able to open any drive by double click.
no external drives can be opened by double clicking.

Solution:
1. Boot your system in Safemode
2. Open your flash drive via Command Prompt (do this via Start->Run->cmd.exe).
3. Change your logged drive to your USB flash drive
(e.g. if your flash drive is at drive E: then type E: on the command prompt then press enter)
4. Type ATTRIB -H -R -S AUTORUN.INF then press enter

After reboot, you can now access your drive.