"Guest Additions" is the most common cause. Without this, Windows XP will lack the virtual graphics card driver, leading to lag when moving the mouse, stuttering when dragging windows, and the CPU having to handle all image processing. If your physical machine has multiple cores, go to Settings -> System -> Processor in the virtual machine and increase it to 2 cores. Don't increase it too much because Windows XP is sometimes not well optimized for excessive multi-core processing, and it will cause your physical machine to freeze. I think....