changing root shell

[Anish Muttreja <muttreja@Princeton.EDU>]

Hi everybody
I did something very silly on mu laptop. I manged to set the root shell to 
an incorrect value.
Now I can't login as root. I know idiots like me should never be root and 
the world is probably  a better place this way , but then neither can  
anybody else :-D.
Short of booting the laptop with a knoppix cd and correcting /etc/passwd,
Is there any other hope ? Basically is there a way to overwrite 
/etc/passwd.
I tried su --shell=/bin/bash .That says No shell and exits.
I will really appreciate all suggestions.
Thanks
Anish