While using Mail, I got a message that there was an error with Mail and I needed to quit and reboot. When I re-opened mail, it prompted me to set it up and while it was reading the files, it froze. Subsequently, my Imac (OS 10.6.3? Snow Leopard) became frozen, i.e. my whole computer. I shut down using the back button (nothing else worked) and then restarted. I then re-booted using the regular re-start system. I was able to do 2-3 functions (the computer was super slow) and it froze again. After 2-3 repeats of the same procedure, it freezes after being turned off and on using the back button. I updated my software about 3 days ago. There was a major security update in that update. I'm waiting for a professional techy to call me back, but if you have any suggestions on how I should proceed, I'd be most grateful.