Hi there,
If it's within your budget, consider a 256GB SSD for the system and OS drive. Then use the older secondary HDD for file storage. What is the size, configuration (Satat II, III, Pata) of the reusable drive?
To specifically evaluate the older HDD condition, consider downloading the free version of HD Tune on a laptop or other computer, then connect the HDD as a secondary drive, to see if there is any physical problem or S.M.A.R.T criteria that would indicate a problem before going to all the time, and frustration, of a clean Windows OS install.