Could you post back with a screenshot
of your Disk Management (diskmgmt.msc) window showing your full HDD layout? This will allow us to give you a more accurate answer.
For now, I would say disconnect the HDD when you are ready to install Windows 7 on the new SSD.
Afterwards, connect the HDD back, delete or format only the partition that Windows 7 is on. If you delete it, then you could extend another partition on the HDD into the "unallocated space" to merge them.
Let's see what the screenshot shows before doing anything though.