That link also gives a walkthrough of what different ratings mean.
To the OP, is your computer recognizing everything you put in? Particularly the RAM? People have been having problems with Vista recognizing full amounts of installed RAM. Your computer should get a pretty damn high rating.
I have seen problems with Windows Experience index and HD 5870 cards. It's most likely drivers. Unfortunately you didn't tell us which one was getting what score, but I'd recommend downloading all the drivers off manufacturers websites (e.g. AMD for your 5870) instead of trusting Windows Update. Mine still tries to get me to downgrade my display driver.
have you tried to do the rating again?? if you havent windows rated it in the end of your windows installation when you didnt have video card drivers installed so do the rating again. if that isnt the case i dont know what is...