How best to optimise my gaming PC?

So I've been meaning to ask around for suggestions on how to optimise my PC. I know my way round my PC for the most part but I'm out of ideas since my favourite Steam game stopped working and my PC seems to give up. I tried monitoring my CPU/ram usage and it was generally at okay levels as long as nothing else was open. My second monitor usually gets switched off too, plus the plug pulled out the back for added measure. But now it refuses to run the game even with no other programs running, just one monitor and the game on basic graphics settings.

Can anyone suggest any ways I can improve my performance for gaming, ideally without buying anything new? Product suggestions are welcome too though. Below is my PC spec:

- Windows 7 Ultimate 64-bit service pack 1
- Intel Core i5-3570k @ 3.40GHz 3.80GHz
- 16gb RAM
- AMD Radeon HD 7800 Series. 4gb dedicated memory.
- Monitor one 1680 x 1050 (32 bit 60Hz)
- Monitor two 1360 x 768 (32 bit 60Hz)
- Seagate Barracuda 1TB HDD
- 750w power supply


I should also mention my PC gets left on for long periods of time (weeks), so some of the components are ageing quicker. It was a custom build which I properly finished in early 2013, so approaching 4 years old now. The power supply is about two weeks newer than everything else since I may or may not have accidentally short circuited the first one :ninja: