Illegal Operation
 

I am try to input some data onto an Excel worksheet but
after I have typed about 3-4 lines I get a "this programme
has performed an illegal operation" and I have to close
the programme which means I loose everything I have just
typed. When I try to go back into the worksheet I am told
that it is already in use. The only way to clear this is
to re-boot. Does anyone know what is happening?