It may not be the game, as such, just something else that's running in the background that becomes apparent while you're playing the game. I'd do a long search for a virus or spyware.
I've tried that today, did a big virus scan and everything came up clean, spyware was the same way.
I've even tried disabling background tasks because I know that Nortons can be a resource hog when it updates, but it's definitely the game that is sucking the resources....I opened up my system info while I had the game running to check it, and the CPU is flicking constantly around 90+ in the field for the game's process.