[razputin]

If it played ok once, then it can again. Based on your description, I would suspect that something external to the game has changed and is causing the problem. Is it possible that Windows performed an update or another program automatically updated something? Did you update your display drivers?

Are you talking about a laptop or desktop? If it is laptop, then the CPU speed is generally throttled up and down depending on the power settinsg and/or the BIOS, so you may want to switch to a desktop with a discrete graphics card.

You might want to try shutting down all programs running in the background (use Windows Task Manager) as well as in those visible in the task bar.

Make sure that you have enough memory available (also visible in Windows Task Manager) otherwise your system might start using hard disk space which could cause a decrease in performance.

Defrag your hard drive. Perhaps Windows moved some files around during shutdown or start up.

Re-install the demo.