How do I Get Dell Optiplex GX1 to recognize 40 GB HD?

I bought this optiplex GX1 for internet browsing, it has a 5gb hard drive as c:, I installed a 40 GB hard drive and whne I go to format it, the system reports it as a 5gb, no more than that, tried othe Hard Drives, a 160gb, same thing, a 5gb threshold…
Any ideas, suggestions?

Thanks In Advance!
Shane