Having added a second HDD to my Dimension 4700, what next?

Hi

I posted recently about how to add a second HDD to my 4700.  I have now installed a WD 320Gb disk, wired it up and added it at the system set up screen so that when I look at the system on Windows it shows me 2 HDDs, which is right - the old one and the new one.  But when I open "my computer" this only shows one HDD - the old one.  There must be something basic that I am not doing!  Any suggestions, please?