Thanks Jason, have sent this to the client. So far, no news is good news.
The one thing about this problem that was puzzling us was what caused a sudden change in the logging into the site about two weeks ago.
It appears that this was probably caused by a windows 7 automatic security update that went wrong and didnt complete properly and this took place on 14/15th August 2012.
This update appears to have caused a lot of problems for some windows 7 users.
I wonder if anybody else's clients reported problems with login or site function after or since this update.