Actually that particular problem does not appear on my 64bit Win Vista on IE8 or IE7. I have used a Windows 7 OS before for testing code, and most likely I tested some jQuery code too (including animation), and have never had any issues such as yours. The only thing I could think of, it would be hardware support (just a guess).
If I were you I would re-install IE8 just to see if it makes any difference and would try the animation code that you have in some friends' computers to see if the issue appears in those PCs (if the issue appears in other PCs, you might be the one that discovers a bug in the jQuery's animation function in Win 7 OSs [well the "bug" could be in the OS itself] ).
As soon as I get my hands on a Windows 7 OS, I will test the animation function again and will post back here.
You can ask the question at the jQuery forums too, the might already know about this.