You may be just reading the wrong figure in Disk Utility, have a look at the quote below from the apple discussion forums
Disk Utility will show two different
"free" readings, depending on where
Sounds odd, but they're for different
purposes. The larger figure, which the
Finder and other things will use, is
the total free space available.
The second is the amount of free space
available for a new partition. It's
shown when you select the top line for
the drive, click the Partition tab,
then select a partition, the
"Available" amount may be smaller.
This is because partitions, unlike
folders, must occupy contiguous space
on a disk. The data on your OSX
partition is scattered over it, not
all nicely packed together. That's
normal, as you add, update, and delete
files, not every nook and cranny is
used again immediately.
But whether there's 7 GB free or 18,
that probably isn't enough to be
adding a partition; it's probably too
little for OSX to operate well.
There's no "hard and fast" rule, but
you should always have at least 10 GB
or 10% of your OSX drive free; many
folks say 15% or more.
You could back up your drive (on two
different HDs, to be safe), reformat
it to 2 parttions, then restore the
backup. But it's going to be very
tight, and you'll likely have
performance problems because of it.
Try to delete anything you don't need,
and/or offload stuff you don't use
often and delete it.