You disable HyperThreading in the BIOS of computer. You might need to check the manual for your particular PC or MotherBoard, but you normally get into the BIOS when the computer first starts up (usually counting memory or showing drives being detected) you most commonly use the DEL key to getinto the BIOS but on DELLs and other computers you might hit F2 or F12 for that set up.
In the BIOS, the HyperThreading option is usually in the CPU or PROCESSOR settings page. It should be as easy as an item that states HyperThreading (enabled or disabled)
In this thread here on this forum, this person talks about forcing Windows XP to only use 1 CPU, but I am not sure if this disables HyperThreading or not:
http://www.satelliteguys.us/showthread.php?t=52290
I would stick to the BIOS change myself.