Inconsistent behavior in minibuffer

[Will]

Hi,

if I open a file C-x C-f
then C-0 C-k deletes the path but not "Find file".
but C-a jumps back to the Column 0 (before "Find file") and marks the 
line without that I want that. So what is the beginning of the line?

In a larger context it is inconsistent that a buffer can be read-only
(cf. "Find File" prompt) and editable (cf. <path>) at the same time.

I am working with
GNU Emacs 22.1.1 (i386-mingw-nt5.1.2600) of 2007-06-02 on RELEASE
on MS Win XP.

Best, Will