I'm running Celestia on Windows XP with some S3 graphics card (that I don't think matters.)
Everything was great until I started the Demo and stopped it midway through - somewhere around the Sun, I think. Now when I use the mouse to reorient myself (left button) and accelerate, I never seem to go in the direction I'm pointed. The X key, which I thought was supposed to fix this, doesn't do anything. Time is normal (real time). And I'm not Tracking an object.
For example, I can Center on Earth, hit X, and still go flying off to the side.