My laptop has Vista Home Premium and came with a 100GB HDD partitioned to two
drives. The drives were to be reassigned new letters, but accidentally only
one partition was assigned a letter. The other drives letter was cleared but
a new letter not assigned. Now "Computer" only sees the drive to which the
letter was assigned.

I cannot use restore as there have been a number of updates before this was
discovered and a restore point prior to this event isn't available.

Any suggestions please?