Try the install disk and boot into rescue mode.
You'll be able to edit grub.
Follow the directions.
Comment out the new entry for PAE and allow the old entry to boot.
If vi i the editor- eg vi /boot/grub/menu.lst then esc+i inserts a character. The pound sign hould be inserted just before the pae kernel entry before each line.
Something to the effect of:
< line of text, follow commands, mount directory>
<at Title do the following>
<esc+i> # <esc>
<do the same for the root and kernel entries to this>
<type :w! and hit enter then :q!
Exit from rescue shell and reboot.