First, find out what your windows 7 product key is. What version of windows 7 is it? E.g. Windows 7 Ultimate, Professional, Home Premium, etc. And what bit system type as well. All this is under "My computer>Sytem properties".
Then, you could download your version of windows from a site such as this:
http://www.mydigitallife.info/official-windows-7-sp1-iso-from-digital-river/
It is legal (pretty sure).
Download that to a bootable flash driver or burn it to a dvd. Install that onto your SSD. Use your product key from your HDD for your SSD.
Plug in your SSD and your HDD. Go to the BIOS, select SSD as boot drive.
Your computer will boot to the SSD, run from the SSD but all your files from your HDD will remain. You might have some shortcut problems but you should be able to go into "Start>Computer>HDD drive letter(?)>whateverprogram/fileyouwant".
And if worst comes to worst, just restart the computer and boot from your HDD. You might have to re-enter your windows product key.