Thanks Gilly - I turned off the screensaver (as Caroline suggested) and managed to get the de-frag to work yesterday and it does seem a bit quicker today.
Have decided to be brave and have a go at putting some more memory in and see if that solves the problem. Did the check on Crucial, as you suggested Gilly, and have ordered it from them.
I read in one of the computer mags that you need an antistatic wrist strap when putting in the new memory - is this necessary?
Thanks for all your help Gilly x