Ever since I bought my AMD Athlon 64 X2 Dual Core, I've regretted it. Don't get me wrong: I love AMD chips, and have never had a problem before this one. Turns out, that the AMD Dual Core just doesn't play nice with a LOT of things, and this just KILLS all kinds of applications, games, movies, etc.
The typical problem is this: you start an application, and it appears to run very very slowly, or jerky. You get impatient, and pull up the task manager, which takes FOREVER. You discover that your application is utilizing 100% CPU ON BOTH CORES!!! DAMN!!!
My most recent problems have occurred within a cool game called "The Movies", and in Microsoft's XNA Game Studio. After doing some searching, I came across something called an AMD Dual Core Optimizer. The heck is that? Apparently, it helps control the behavior of the chip, when software vendors decide to go around the windows API, and do direct calls (which apparently is the reason for the issue).
Well, I can tell you, I installed this puppy, and all is wonderful again! My XNA is working (mostly), and "The Movies" appears to be playing nice again. If you have an AMD Dual Core, I recommend taking a look at the link above. Be sure you know what chip you have before installing anything though: I can't be held responsible for anything that you might do wrong!! (whew, disclaimer out of the way)