In windows XP, (I think Vista, 7 and 8 too), a hyper-threaded single core CPU running a single thread at 100% utilization would show 50% CPU utilization in windows task manager. To extrapolate, this would say that your computer using a dual core with HT would show 25% when a single core is being utilized 100% by a single thread; that is if your game is single threaded, it would be use 100% of a core, which is 50% of the CPU, but only show 25% utilization.
If you disable HT, the single threaded game will still utilize a core 100%, which is still 50% of the CPU, but it will actually show 50% utilization.
Many articles have shown that having HT enabled on average increases performance even though there are circumstance that HT shows decreased performance. That being said, the biggest advantage of turning off HT would be so it feels better because it will look like you are using your CPU more. Psychology would say via the placebo effect that this might actually make it seem like there is a performance benefit because you feel like 2x as much of the CPU is being used. While many might disagree, how you perceive things can have more of an impact than how things actually are. If this is true with you, then turn HT off! The performance decrease would be minimal.
But if you want to stick with the facts, then disabling HT would decrease performance on average regardless of what the CPU meter says, there in you should leave it enabled.
As for whether you can enable and disable HT on that computer; it looks as though in the user manual for your laptop there is nothing about having options in the BIOS that you can modify. You might try a BIOS update to see if that allows you to toggle the option. Other than that, if you cannot modify the setting, you are pretty much out of luck.