I'm at work and thought I would throw this out for ideas... Working on Macbook Air 13-inch Mid-2011. It had a virus and we decided just to wipe it and reinstall Lion. We work behind a firewall with a proxy so I tried to use a Lion USB installer that I often use. When booting holding option I see the installer but as soon as I select it I get the prohibitory sign (no smoking sign). Can't use internet recovery because of the proxies (big company, no turning them off). Now trying to make a dvd installer from a lion DMG but just wondering if anyone has ideas. I've already wiped the disk, made a new partition, and done PRAM reset. Why am I getting the prohibitory sign? Thanks.