hmmm.....if the overwrite came short of the virus or was the same size as before, I suspect you would still be ok as there would be no pointers to the virus to execute it and there would be some sort of "End of File" marker at the end of the valid code, once again stopping the virus running.

Like when you wreck LILO because windows somehow clobbered the start of the partition, Linux is still there, but theres no pointer to it.