Have had a HP Pavilion 520c since 2002 and for the most part I have had reasonable success. However, since the HP warranty has run out, I have decided to clean everything HP out of the computer, add a second 80 gig drive and reload with a retail version of Win XP Home with the Service Pack 1 package. The computer has a ASUS A7V-ML 1.09 mother board. My question may sound simple but I can find no technical details on this board. It has 3 PCI slots (ivory colored) and then one more shorter socket (black). What is this slot used for?