I appreciate this is a common problem with various threads already posted, but none them has come to my aid so far, so forgive me for this. Basically, I got a new anti virus programme from the App Store and it showed that I had that new Backdoor malware thingy on my 2011 Mac. I deleted all the files which the anti virus considered a threat and since then I haven't been able to boot up. It started with the blue screen. Then I tried the hold shift thing and it got to the plain Snow Leopard wallpaper but I couldn't get anything else to display/work. Then I tried the hold Alt key and that did no better, but it did show me the option to click on the HDD, which I tried. I have some files I haven't backed up that I'd ideally love to retrieve. Of course, if possible I'd like things to go back exactly to how they were. I have the OS X install DVD but haven't done anything with that so far because I'm in above my head now. I'm a little panicky at the moment but if the worst come to the worst, I'll just have to heal the wounds with a 2012 Mac that I previously had no intention of buying!