Multi-core CPU systems, need a help

I have a single threaded application working under 3 different multi-core
systems, one with XP 32 bit, one with Vista and one with Windows 2003.
On XP the workload is all on a single core, while in Vista and 2003 the
workload affects both the two cores.
Am I correct in suspecting the 2 latter OS'es (Vista and 2003) are
internally splitting the single thread between more cores?
And if so, is there a way to tell Vista (or 2003) not to use this feature so
to let the single threaded application work on one core only?

[sorry for re-posting but I didn't know of this group when I first sent this
to ...vista.general)

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