There is no such thing as erasing a HD. Every sector will contain either a 0 or 1 and neither affect the data which will overwrite it which cannot even see this.
There are only two reasons I know to wipe a HD:
1. Using Diskpart Clean All command will overwrite the data underneath with zeroes so it is nearly impossible to recover using normal methods. This is for assuring data is not retrieved and not for preparing the HD in any way.
2. In some install failures or other corruption use the Diskpart Clean Command
(quick method) to overwrite with zeroes corrupt or conflicting code in the boot sector. In almost all cases, simply deleting all partitions using the booted DVD's Custom install Drive Options should be sufficient. Then you can partition and format as you wish, or just click Next and the installer will create and format one large install partition for you. Clean Install Windows 7
Follow these steps to get a perfect Clean Reinstall - Factory OEM Windows 7
which are the same for retail.