Hi Erika,
Assuming you purchased and installed the card into one of the 2 matching matching MoBo's PCI Express 2.0 x16 slots, make sure it is seated fully. Make sure you've got both power cables connected to the card.
Two power connectors are on the upper right corner of card.
Next, you'll need a 450W rated power supply unit (PSU) in the PC as MINIMUM. If yours is a "run of the mill", "no name" or "oem" type, it is probably inefficient as the day is long - that means bumping up to a 550+ watt rated PSU instead.
Lastly, have look at page 3-1 of the
manual. Look at the table that describes POST codes. if it will not POST, reinstall the GeForce 8600 video card and go for boot. If it works, I suspect that your PSU and / or power connections are lacking.
I hope this helps and good luck! Please rate my reply - thanks!