You should not have to bail out of the install if you install a second drive and set that as the boot drive in the bios.
Your new drive will have the default partion of C: under windows 7 and the other partions can be set to what you want.
Any partition can be assaigned any disk letter you like using the disk manager in administrative tools under 2000/xp/vista/w7 - the problem you have is the primary active partition is not able to be changed as it has the operating system on. IF you can set in the bios to say it is a secondary new disk then you can perform a new install on that disk without disturbing what is already there - and then use the disk manager to set what was the D: Drive back to being D: once done.
It all depends on what you want to do - you can install windows 7 and destroy what is there (no need to quit out of it as thats one of the options) or you can fool it using the bios into thinking it is a clean install.