Macbook Pro "Your computer needs to restart" error

There is a very long discussion of this problem here:

https://discussions.apple.com/thread/4278087?start=0&tstart=0

and here

https://discussions.apple.com/thread/4196488?start=0&tstart=0

and here

http://www.betalogue.com/2012/09/24/mountainlion-kernelpanicsolution/

Thanks for your suggestions.
Am working through them and am having difficulty in booting into Safe Mode.   The instructions are not exact. I take it that you should:

Close down the computer.
Start up.
When you hear the "musical" tone (not the disk starting). Hold the shift key down -

Is this either shift key. Have tried with the left hand one (above the fn key). Is this correct?

Do you then release the shift key when you see the apple logo AND the spinning wheel or just the Apple logo?

I cannot get it into Safe Mode am wondering if I'm misreading the instructions.


When you see

Hi strung,

Think I have found the reason.
When this happened I was using Pixelmator (the marquee tool). Have discovered this:

Pixelmator problem

Your suggestions, however, have been very helpful and, when I have discovered how to boot in safe mode, will run though them in case there is another reason.   Before I assign the points, can you suggest why I cannot get into safe mode?

Brian

Thanks, strung.