First off, a big thanks to all helping me out here. I didnt expect such a good response - restores a bit of faith in humanity.
Right, I've backed all my stuff up, and any fatalities now will just mean a loss of time rather than my precious precious data.
My 2 hard drives are a 120GB (WIN7 OS) and a 2TB (My Documents and back-ups etc).
The 120GB drive is about 4 or 5 years old now, whereas the 2TB was bought just a few months ago.
I have attached two screenshots for the reports from HD Tune Pro 5.00. The health status on the older drive is 'warning' and there are two yellow lines there too. I'm no clever scientist, but it looks like there might be problem with the drive.
I think my best solution here is to clear all the stuff of my 2TB onto my external HDD - Reformat it and put my OS on it and then chuck the 120GB in the bin?