Most times this has to be done by entering the bios. Usually you can do this by hitting delete on startup or one of the function keys. Some Dell computers do not allow access to the hyperthreding option in the bios, just FYI. Also, I did not notice a performance increase by disabling hyperthreading, although others have.
I have seen threads at many various forums, etc, which basically confirm what Joe said.. I simply wanted to reiterate that you can read plenty of support for both disabling as well as leaving it alone, on xp.. leaving one such as myself( dependent on the findings of many,reflected online on forums and such as this one) even more reliant upon wise tinkerers who are willing to share and without bias do spread a valid truth for us to truly learn from.
And so I have been watching this subject for many many days&nights in every
place I see it arise, this discussion is one whose result I am infinitely in search of.
best of luck
Windows interrogates the hardware on setup. These items are then added to certain areas of the registry. This is why you need to shutoff items such as hyperthreading in the bios. You can also control how windows handles items such as irq (interrupt) settings by making changes to these in bios. The bios is the link between the hardware and the operating system.