Hmm, have you recently cleaned out your PC, like the registry, and deleting tempory files, etc? These tools can be found under Start, Programs, Accessories, System Tools.
Also, after doing that, try a Defrag from the same place. To get the best results, run in safe mode (Press Del at bootup and select it from the list)
This will sort through cirtical system files, moving them about to make them easier to access, so your system should be a bit faster.
Now try playing the game. Any improvements?
Also look for programs in the taskbar that you don't need that are running as standard when Windows starts that you can stop.
You see, your computer does get slower after a while, and do worsen as time progresses, so I wouldn't worry. Hopefully these checks will get it going a bit faster again, though nothing helps better than a full format and re-installation of Windows. If you were to decide to do this, you must make sure of course, that everything is backed up
I hope this helps