My processor is nearing 2 and a half years old now, so is my 2gb of ram, although my graphics card had to be replaced a few months back with the latest model (at the time) a radeon 4870 HD, also known as a central heating system for your PC in some circles due to how bloody hot it runs and how many problems you have with it. And i'm on XP. And i haven't defragged in months, and i haven't run a spyware or adware or virus scan in over a year, mostly because i know what i'm doing and don't need to, but still.

All in all, your PC is vastly superior to mine, yet performance for me is grand. Everything on high, shadows turned off (as i do in all strategy games... they're useless unless you're playing FEAR or DOOM 3 or games of that nature, at least imo), and unit size on large (120 men in a line inf unit), but i did have them on huge, i only turned them down because i was told it helps the AI do better in battles. I never bothered to check what FPS i'm getting but it's as smooth as a babys bottom.

So yeah, it's mind boggling.. Definitely not your PC that's causing the problem... simply... bad software.