Blender is running slow.

Hi, I downloaded Blender for my PC and it is running Windows XP. And I am creating a 3D bullet to put in a movie of mine. And when i downloaded it, it was working great and fast but then all of a sudden it started to run really slow, and I don't know why. I tried scaling the bullet down and it took like 5 minutes to show me what I changed. And it goes for everything. And I also downloaded it for my new Apple Mac Book Aluminum Pro and it did the same exact thing. And i also tried re-downloading it. What can I do to resolve this?



      

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