Welcome to the forums, Mark.
It does depend on where you install 7, it'll delete everything if you do a clean install on that same partition.
What I personally would do is to install my hdds, one time and forever, connect them in a RAID if possible, allocate several equal partitions, and finally clean install 7.
In your case you may want to backup your personal data first (as you say you have).
According to Microsoft:
There are two options to choose from during the Windows 7 installation process:
Upgrade. This option replaces your current version of Windows with Windows 7, and keeps your files, settings, and programs in place on your computer.
Custom. This option replaces...