Yes, you can disable Hyperthreading in the BIOS, but why would you? Hyperthreading makes the CPU perform better. Is there a reason you're wanting to do this?
Edit: If you're doing this because you have a single threaded app, don't bother. It won't make it run any faster. If it can max out one physical core then disabling Hyperthreading will not make it run faster. In fact, it will hurt performance because the system will have less options on where to assign background tasks.
No that program isn't running any faster now, it's just that now windows sees your processor as a dual core instead of a quad core. One core running at %100 out of two is %50 of your total processor power but if you use hyperthreading it still runs %100 of that same core but now you have 3 threads idling thus windows sees it as %25 of your total processing power.
To put it even simpler turn hyperthreading back on. It will help you, trust me
You could also go into Task Manager and assign the affinity to only use the first core (CPU 0). You just right-click on the task in Task Manager, select "Set Affinity" and un-check all but CPU 0. That way your task would only use the first core and you wouldn't have to cripple your CPU in the BIOS.
Edit: Paigeinfull is right if the task was already maxing out the first core. I bet if you time whatever task you're performing with Hyperthreading enabled and again with it disabled you will see that it's slightly slower with it disabled.