More detailed post ...

[Dave Selby <dave_arahan@yahoo.co.uk>]

I either get 

Unmounting local filesystem ... done
rebooting

and all is AOK, or I get ....

Unmounting filesystems ...

and the system hangs !!!, ie no done !!
My guess is it fails to unmount the file systems.
my fstab is 

# /etc/fstab: static file system information.
#
# <file system> <mount point>   <type>  <options>               <dump>  <pass>
/dev/hda2       /               ext3    errors=remount-ro       0       1
/dev/hda3       none            swap    sw                      0       0
proc            /proc           proc    defaults                0       0
/dev/fd0        /floppy         auto    user,noauto             0       0
/dev/cdrom      /cdrom          iso9660 ro,user,noauto          0       0
/dev/hdb1       /mnt/archive    ext3    defaults                0       0
/dev/hda4       /mnt/debmain    ext3    defaults                0       0
/dev/hda1       /mnt/win98      vfat    defaults                0       0

my mtab is

/dev/hda2 / ext3 rw,errors=remount-ro 0 0
proc /proc proc rw 0 0
devpts /dev/pts devpts rw,gid=5,mode=620 0 0
/dev/hdb1 /mnt/archive ext3 rw 0 0
/dev/hda4 /mnt/debmain ext3 rw 0 0
/dev/hda1 /mnt/win98 vfat rw 0 0

all looks OK to me except when it goes wrong, no hint or clue in /var/log
Dave