Did you do an "in place" upgrade to Win 8 from a prior version of Windows or did you reformat the hard drive and do a clean install of Win 8?
There was a similar problem with an XPS 435mt recently where the user upgraded from Vista to Win 7. Windows said there were 6 GB installed with only 1.4 GB usable. The solution there was to wipe the drive and do a clean installation.
If you decide to do that, you must back up all your personal files on external media (USB hard drive, DVDs ,etc) because everything will be lost. And you'll need the disks and product keys for all your software (eg, Microsoft Office) in order to reinstall after installing Win 8.
You can find a useful guide to installing Windows on a Dell PC here.
EDIT: I fixed the link.