I have now twigged why the renaming took place. It's because I left the old (F) and created another partition on it by installing the 64bit OS on the same HDD.
I should have just reformatted it and installed on to a clean HDD.
Unless I can just change the letters I will disconnect the 2nd HDD and get the orginal (C) back to normal, and worry about Win7 later.
I will however, make a mug of tea and reflect the meaning of life in case someone comes up with an easy solution - if there is such a thing...