To get an idea of what the partitions are try running -- as root --
"fdisk -l /dev/hda" (assuming your drive is hda, which it probably is).

Then to see how the partitions are being used try
"cat /etc/fstab".

Why don't you post the results of those two commands so we know what you're
looking at. Depending on how things are laid out it may be fairly simple to
use "parted" to expand your current partition(s) into the unused space.

-Steve