It's easy to check to see how many cores are detected. Just use task manager and look under the performance tab. If two cores are present, then task manager should display two boxes under CPU usage history with each box graphing the workload of the cores. If one is disabled, then one long rectangle is displayed instead graphing the workload of only one core.
Here is an example of a single cored computer in task manager:
And here's and example of dual core. The picture is actually one of hyper-threading, but yours should look like this:
::The OS only detects what the BIOS detects. Check BIOS settings. You may have altered some things when installing XP. Also, what are you using that tells you one core is not detected?
The picture below shows a dual cored processor in Task Manager, if you do not see two black boxes, then something is really wrong.
11:22 PM by