You should try to update your FX5700 drivers to the latest one as well as getting the latest dx9.0c build.
The lastest DirectX build is "august 2007". You can get it here:
Even if you have DirectX 9.0c, every month (or so) there's an update (mostly bug fix and improvement, but no new features) and even if you have auto windows updates enabled, it don't update directX so you still have to sometimes check for the latest directX build.
The fact that the game installer try to install directX every time is maybe because you don't have a so up to date directX build.
The latest nvidia drivers are the Forceware 162.18 for Windows XP and 162.22 for Vista only. You can find them on nvidia website.
If it still doesn't work, check if the XNA runtime redist are not corrupt (download and re-install XNA runtime).
As a general advise for everyone having trouble running a game or an XNA tutorial, always check if your drivers and directX are up to date. I know that most of you know about having those thing up to date, but some don't.