Hiyya Sue your board is this ASUS - Motherboards- ASUS M4N78 PRO
You can see from the different sections - Specs it has a Nvidia GeForce 8300 and in Downloads the download for the Chipset which includes the NVidia Chipset package (make sure you enter the correct OS system to find the right download).
There are three different download sites I usually use Global as I am not ken on the P2P stuff.
Now don't take offence but as a tip most times mate it is real handy to post your system specs or at least have them listed in your public profile - it does help anyone trying to help other (you are not alone in that).
Things like the OS CPU machine model or the board you used amount of RAM etc
It was lucky you mentioned the model so it made life a tad easier.
Just as an afterthought have you checked the HDMI cable itself? They are flimsy things really. I use DIV and a separate audio wherever I can as I have had problems with the cable not making good contact or even at all. Plus they are prone in my mind to slipping out .
Let us know how you go