Your lowest cost and most effective change would be to change to a ssd.
My recommendation is to buy a 1 or 2tb samsung 860 evo sata ssd. 1tb is about $100
Use the samsung ssd migration app to move your c drive to the 860 evo.
Here is a link to the manual and app:
Download various software related to your product, including Samsung Magician, designed for use with Samsung's memory products, and find related information.
www.samsung.com
You just run the app to copy your c drive installation to the ssd.
Then, just change the boot order.
After, you can repurpose your HDD for whatever, backup storage would be a good idea.
Everything you do will be quicker.
The ssd will be some 5x faster in sequential reading than your current hard drive.
It will be 40x faster in small random I/o which is what windows does mostly.
So far as cpu power is concerned, there is not much to be done
without changing to a modern ryzen or intel processor.
That will also involve changing the motherboard and getting ddr4 ram.
I would do the ssd first, since it can always be moved to a future new build.