XP is an operating system. It cannot be "multicore" - Multicore is a hardware term describing a CPU, in particular, a CPU with two or more "cores" that act as individual processors.
As for XP, it is MULTITHREADED. In order for applications to use multiple processors, the OS must support it and the application itself must support multiple threads. XP uses preemptive multitasksing to dish out processor time to the running applications.
Then, assuming you are asking this in part because you have performance concerns, the performance of a system depends on a variety of components, including the slowest vital component, hard drives.
Finally, NO OPERATING SYSTEM scales CPU performance without SOME performance hit. Meaning that a 4 core CPU system (linux based, unix based, or windows based) will NOT run an appropriately multithreaded program 4x faster than a single CPU core - in ideal circumstances, it MAY improve the application's performance by 3-3.5x.