Hi Nige,
Right, regards Clives question regards virus scanning: You say he scanned with Norton, but if he does not connect to the Web, his Virus Definitions will not be upto date.
Regards his missing 30GB, what does "System Info" say regards its size?
Right click on > My Computer left or right Click > System Info, on the System Tab down the bottom it should say something like 37.31GB
I'm on a 40GB HD, and thats the figure it shows for mine.
Sorry I'm off to make tea now