You're walking a bit of a fine line here with the licensing.
What it comes down to is how Microsoft defines a PC. Typically, they define a PC by the motherboard used in the unit, as Windows does not automatically have to re-activate itself right away, until the motherboard is replaced. Other components can be swapped, upgraded or added (newer CPU, more RAM, extra peripherals like a sound card) and Windows doesn't care one way or another.
Now I am not affiliated with, do not represent, or speak for Microsoft in any way, but I believe you will be ok, so long as you wipe out your current hard drive after the SSD you want is in your hands and ready to be used. Once Windows 7 is installed on your SSD, you must remove the Windows 7 install from the old hard drive to avoid blatantly violating the license agreement.