Ashampoo's Core Tuner ($20, 10-day free trial) promises to boost your multi-core PC's performance by tuning XP's or Vista's usage of multiple CPU cores for individual applications. Perhaps it does. There are inefficiencies in the way XP and Vista handle multiple CPUs, though this has been fixed in Windows 7. In appearance, Core Tuner has the aspects of a fancier version of Window's own Task Manager with a drop-down menu that allows you to set each applications priority to low, below normal, normal, above normal, or high. There's an auto-optimize option as well that lets Core Tuner do the thinking.
Bottom line, I was unable to spot any difference in performance in Word, Outlook, or any other application set to high priority on either my Intel Core 2 6600 or AMD Phenom II 9550 with Core Tuner. I was able to perform only subjective testing so there may have been a performance boost that simply wasn't measureable by the naked eye. That said, if you need a stopwatch to notice the difference, any improvement may be too minor to mention for standard PC use. My guess is that during a normal workday, Core Tuner's performance benefits will never make up for the resources and CPU cycles utilized by the program sitting in the system tray. That said, the program behaved itself, so there's nothing to stop you from benchmarking it yourself.
Caveat: You'll want to be careful downloading the Core Tuner free trial if you do so from Ashampoo's Web site. The Softonic site Ashampoo points you tempts you with $3 faster, always available download option--which is completely unnecessary. Plus, the page that comes up after you click on the free download button sports one of those advertising download buttons you might be fooled into clicking before the real download dialog appears. The whole process is disingenuous at best.