|When Hyper-Threading is enabled in the computer's BIOS, Windows XP automatically upgrades the hardware abstraction layer (HAL) if it must use a multi-processor HAL, and an additional processor or processors may be installed and listed under Processors in Device Manager. The system will prompt you to restart so that the new settings can take effect.
The above quote was taken from here
Essentially, when HT is enabled in BIOS (as you have done) WinXP should automatically start using it without any manual configuration from the user.
If HT doesn't appear to be active on your install of XP after you have made sure to enable it in BIOS and saved the BIOS settings before exiting BIOS, then there would appear to be a problem with your system, or with task manager's display.
Try downloading and installing Speedfan
which should show two bars in it's CPU usage display if HT is enabled. You could also try downloading the Intel HTT Test Utility
which will determine if HT is enabled on your system.
If you find that HT isn't enabled in Windows, but that you have indeed selected it to be enabled in your BIOS, perhaps it is possible that you don't actually have an HT CPU..?
Hope this helps.