Installing 8800 gts on hp computer

Before you took the old card out, you should have uninstalled the drivers, and reinstalled the standard Windows VGA drivers. Now, it makes no sense when you say that the card does not work, because it installed the drivers for onboard video. How in the world did you boot into Windows and install drivers if the card was not working.............
Anyway, I don't understand exactly what you have going on right now, but here is what you should have done.
1. Uninstall the current drivers, and install the standard Windows generic VGA drivers.
2. Delete or remove the old drivers from the system completely using the add remove programs, or a program like driver sweeper.
3. Reboot to the BIOS, set the BIOS to look first for the PCIe slot for the GPU.
3 Shutdown, remove old card, install new card, boot, install new drivers.

I first uninstalled the 7300 le that was on the computer (had all ready been physically taken out). I deleted the nvidia drivers from add/remove programs. After uninstalling all that I reset the computer, put in the new 8800 gts, tried to install it with said drivers and after the drivers installed I reset again. When loading it up it did not detect the 8800 gts instead it installed the onboard 6150.

Ok I started over and followed the directions ( I think) of what to do. I turned on the PC, uninstalled the nvidia display drivers through add and remove programs. Installed standard windows vga drivers. Rebooted into BIOS (was all ready checked to start with PCI-E). Physically installed the 8800 gts and restarted plugged into the 8800gts and I have a black screen monitor. The computer is on and running but no video.