Installing vista

I have an EMACHINE that came with vista home premium. On the hard drive, it has a partition D: and that had a bunch of stuff in there, when i was using the vista that came with it, it said that that part of the hard drive I could not access. So I just left it. Anyways, I have a copy of vista home premium without all the emachine software on it, when I load it, it installs fine, but then when the computer restarts it says that there is a problem. Now when I reboot with the vista disc in there, it allows me to re-install windows again, and it says that the d:/ partition is up there first, then the c:/ is the second on the list, is there a way of moving that d:/ down to under the windows drive, or do I have to delete the d:/ partition. The reason why I dont want to delete it ideally, is because if there is a problem with this vista disc that I have, I want to reinstall the one that came with windows.