So you could do it... But honestly, I dont think you are going to be happy. Just not enough space.
With the plummeting prices of SSDs I would hold off. Get a 250-500GB SSD. Trust me, you will not regret the upgrade. It is the best upgrade you can get.
Windows 8.1
If you want to run Windows 8.1 on your PC, here's what it takes:
Processor: 1 gigahertz (GHz) or faster with support for PAE, NX, and SSE2
RAM: 1 gigabyte (GB) (32-bit) or 2 GB (64-bit)
Hard disk space: 16 GB (32-bit) or 20 GB (64-bit)
Graphics card: Microsoft DirectX 9 graphics device with WDDM driver