Hi David, first of all, good to hear your new PC is finally working!! :) Actually it´s not a bad idea to leave the C: partition as it is at 97 GB. This would then be your main OS (Windows 7) drive and used only for that purpose, updates, system restore points, etc. I would then create a smaller partition, say 30-50 GB for programs and applications and the rest depends on what you use your computer for. (Gaming, video-editting, etc.). Of course it´s your computer and you can do what you want with it, hehe.
So, the easiest way, one big C: drive you asked for would be to delete volume D: Go to Control Panel -> System and Security -> Administrative Tools -> Create and format hard disk partitions. From there, right click on D: then delete partition. Then right click on C: extend partition and allocate all of the space to C:
If you want to partition as I metioned earlier then post back and we´ll help you out!! Matt