I would like to add that Vista home Premium is just bad for a gaming machine. That OS was intended for basic, non "resource hungry", apps. It may not add to the reason why your machine is running slow, however, it is not the recommended choice.
Vista alone should have at least 1 GB set aside to run the operating system, even if it is still just "Home" and not "Ultimate".
Check out your Task Manager to see how much memory you are using just running windows on a basic start-up. If you can, increase the PF usage on your memory, that can help with performance sometimes.
Unfortunately, I have not tried to change that on Vista yet and therfore am not sure how...