Hello.
If you just add the SSD and install an OS to it and don't remove the old OS Windows will configure a dual boot between the 2 separate Operating Systems and that will have to be dealt with.
The best way to go is to disconnect the Hard Disk Drive with the "older" OS on it and then add the SSD and connect it to the #1 SATA port on the mobo and set it as the second boot device after the CD/DVD drive in the BIOS and do a clean install to the SSD as usual.
How to Do a Clean Installation with Windows 7
Then when the "new" OS is booting and all is well, power down and reconnect the other HDD to deal with the older OS at will; be sure to post back with any further questions you may have and to keep us informed.