Thanks very much angelwithin - switching off McAfee virus checker seems to have done the trick. I haven't noticed any problems with any other graphic programs though.
Is there anyone there at SBM who can shed some light on this issue? Is it a McAfee issue or is there something that can be done with SBM to prevent this issue? Given my experience with McAfee I wouldn't be surprised that it is the culprit.
In the meantime I will have to disconnect from the net and deactivate McAfee whilst using SBM. But at least I can get my project finished!