You get into the BIOS by hitting a key as soon as you turn the computer on. There are basic instructions here. I think the correct key is F1 for HPs. Once you get into the BIOS just look for an option called "Hyperthreading". It should have two options: "Enable", and "Disable". Set it to "Disable". Then save your BIOS settings and reboot.
You can tell whether you've done it properly by hitting ctrl-alt-delete and then select the "Performance" tab. If you see one box under "CPU Usage History" then hyperthreading is off. If you see two boxes side-by-side then hyperthreading is on.
By the way, most of the time people tell you that disabling hyperthreading may help it doesn't make any difference. Only some old drivers had problems with it. Recent software has to work with hyperthreading enabled or the software developer is in for lots of misery.
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