XP SP2 Athlon X2 Performance BugBy Justin Mann
This is the first I've heard about this particular issue, though I don't own an X2. If you're lucky enough to have one, however, you're probably using Windows XP SP2. Apparently, there has been an issue with using the hardware throttling built into the Athlon64s in combination with the X2 and SP2 that can cause performance degradation. After reading it over, in layman's terms the bug is that a single-threaded application may have its execution moved over to a different core, that core being in "low-power", "low-clock-speed" mode, and it won't properly return to it's full clock speed.
A fix is all well and good - Except that Microsoft is not allowing you to download it at will. You must first actually call them to get the download. A bit rash, perhaps, for a simple hotfix.