Any system consists of an operating system plus system utilities and user application programs.
Most recent operating systems support multi-processor and multi-core platforms. Windows XP will support two processors, each with multiple cores. Similar with Linux and Mac operating systems.
A user system, even with applications written for single core (single processor) environment, will experience benefits from running on a multi-core system. This is due to the fact that the operating system can schedule different utilities, driver activities and different applications to run concurrently on the number of available cores. An example would be burning a music CD while browsing the internet and/or downloading updates from the internet.
vipinco2003:How a system written for single core architecture can take performance gain when run on a multicore platform?