Thanks for the replies folks.
The images are in RAW format, but I have used this format for a long time and can happily download from other cards. The problem exists with my 4GB card only.
I have dowlnoaded the latest drivers to no effect and do not have another Card reader to hand.
Yesterday morning I downloaded some image handling software from Canon which now allows me to transfer the images via a camera to the PC.
I don't like doing this, but at least it proves that the system and card are working and compatible.
I guess I will just have to try and source another card reader for further tests