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[waldronate]

The Windows NT scheduler (XP, Vista) is known for somewhat pathological behavior with regards to multiple threads. What it should do is try very hard to keep a task running on the same core as much as it reasonably can because switching cores involves some serious cache thrashing as it fills the caches on the new core. But the scheduler will often just round-robin a single task though all available cores, leading to sometimes significantly reduced performance. If the total usage on a quad-core CPU stays pegged at around 25% (one core full operation) but individual cores keep changing their amount of work in flight then it's more likely to be the result of the unpleasant scheduler behavior than it is anything to do with the running program. Windows 7 is supposed to be much better in this regard than the earlier versions.
Sometimes you can get a performance boost for single-threaded programs just by changing the task affinity to a single core. The Turbo boost on the Intel Core i5/i7 runs the clock speed higher with fewer cores running so the affinity trick can potentially get much larger gains on those processors than would be expected from a simple reduction in cache thrashing.