I have a 2008 20" imac running 10.7.3, I have 2GB of RAM, and I recently put a new 1TB hard disk in as the old one fried.

Since the replacement, the computer has started to freeze randomly. By freeze, I mean that everything stops working (the clock stops). I am able to move te cursor using the mouse. Sometimes the spinning wheel appears. I have to turn off the machine and start again - it often takes three or four goes to get it to start again. Sometimes it is fine for a week or two, sometimes it stops working after a few minutes. I cannot see any pattern to what is causing the freeze.

I have run disk repairs and it tells me that thee are no problems with the hard disk. I am worried that I didn't replace something properly, and tis may be causing the problem.

One other thing - when I do get the computer up and running and open a downloaded application, I sometimes get the message saying "this is an application downloaded . . . " as if I was using it for the first time.

Any help would be much appreciated.