Opening file at boot time ( read only)

[frank smith <fsmith@walescomputers.co.uk>]

HI
I ve made a mess in /etc/fstab and now cant boot my linux box
I tried to mount my other OS using fstab and now when I boot
It dumps me at the prompt ( ctl + D plus root pword)

When I try Vi the file it's read only even though I use the Root Pword.

How do I access and change fstab with Vi (commands helpful as me and Vi 
dont play well!)

Cheers
Bob