Okay, I decide to take drastic action...
I re-installed Windows 7 (I'd only had it since Christmas, so there wasn't too much to back-up) and unplugged most of the hardware connected to my pc. I re-installed Avast 5 and everything is working fine now, brilliant!!

Therefore, it means that something on my computer, either software or hardware, was obviously conflicting with the Avast memory scan.
I will spend the next few days gradually re-installing all the software and hardware that I had on my pc prior to the Windows 7 re-install and will attempt a memory scan after each install until I hopefully find the culprit. I will then report back with my findings, in case someone else has the same problems that I had.
In the meantime, I would like to thank everyone who offered help, especially Igor, for your time and patience.