If you already have a single hard drive with a C: and D: partition, then you re-size both partitions by 1st shrinking your D: to be as small as possible; maybe Win 7 will even allow you to set it all the way down to zero. Then take that unallocated capacity to expand the C: drive. This can be done using Disk Management that is part of Computer Management within Administrative tools.
Re-sizing the C: is generally not recommended because it can cause data loss, but when I was fooling around with Win 7 for the first time I tried re-sizing the C: drive several time w/o any consequences.
The steps you should do is as follows:
1. Install and format the new hard drive. It should become your E: drive.
2. Cut and paste everything in your D: drive to the E: drive unless you don't really care about the data. This should empty your D: drive.
3. Make an image of your C: drive just in case re-sizing the C: partition messes up Windows 7. I use Acronis True Image Home 2011. Save the image to the E: drive.
4. Click the Window logo in the bottom left corner, go to Administrative Tools, then click Computer Management.
5. You should find Disk Management under the Storage section. Click it.
6. The Disk 0 bar graph should contain the D: drive partition. Right click it, then click 'Shrink Volume'. Unfortunately, 'Delete Volume' will be greyed out... probably because I still have data in it.
7. In the new Windows that pops open, you will be told how many MBs are in your D: drive and the size available to shrink. If there is no data stored, then hopefully it will be the same as the total capacity. Enter that number into the available field, then click the Shrink button. This will take a little time.
8. Right click the the C: drive and you should be able to extend that partition. In the next Window expand the drive by the amount available. After a little bit of time, your C: drive have now been expanded.
Again, while it is possible to re-size the C: drive on the fly, it may damage some data (the OS). I did not come across any issues when I was fooling around with the re-sizing process.