Well I think you are going to have to remove the 7600GT to use the integrated graphics. To check that it's disabled already you could move the VGA connector from your monitor to the connector on the motherboard and start the computer. If the onboard graphics are enabled you should see the initial power up. Most likely it is disabled though since you are using the add-in board. If this is the case, leave the card in until you complete the next steps. Boot into windows and uninstall the nVidia drivers. Reboot and using the VGA output from your 7600GT enter the BIOS (not familiar with Dell's so I don't know which key to use to enter BIOS, hopefully it prompts you). Next find the BIOS page that deals with enabling/disabling hardware and find the line for the onboard graphics. Arrow down the list until you come to it and toggle it to enable. Save the settings and exit the BIOS. Shutdown and unplug your computer (wait for a minute for everything on the motherboard to discharge) and remove the 7600GT. Plug the monitor cable into the onboard graphics out connector and start your computer. Hopefully at this point it should display the start up. You will have to download the drivers needed for onboard graphics (I don't know what graphic chip your particular model has, just look on Dell's support site under your model). You can do this either before or after following the above procedure. Once in windows install the graphics drivers and reboot. If everything went OK, you should be up and running after the reboot.
These are very basic step as I don't know this computer explicitly, so you will have to fill in some of the blanks. Going back to the original setup is pretty much the reverse. Remember to make any BIOS changes using the connector on the device that you have enabled at the time.