Hi Jack.
@Samm's suggestions are pretty good. However, I'd like to add something regarding the "Windows install disk for another computer". Everything that Samm mentioned is correct, but if that disk is OEM (original equipment manufacturer - it came with that "other computer"), you won't be able to use its key to activate Windows on this computer. This will be possible if that Windows versions is a retail one.
Another option for you would be to clone the OS over to the SSD. However, since the SSD's capacity is a lot smaller than that of the HDD, this can happen only if you have the OS on a separate partition, so that you can clone just the OS associated partitions, as there won't be enough space available for the whole HDD to be cloned. You can check out this tutorial: http://lifehacker.com/5837543/how-to-migrate-to-a-solid-state-drive-without-reinstalling-windows.
Fresh install is usually recommended, so if you end up going with that option, in order to use your HDD simply as secondary storage, all you need to do is boot from the SSD (change the boot priority from the BIOS' boot priority menu). It doesn't really matter that there's an OS on the HDD, however, it's advisable that you remove it. The best case scenario would be to backup you important data, format the HDD and then put back everything you've backed up.
Hope that helps. Please let me know if you have any questions.
Boogieman_WD