The Dell Owner's Manual for the C521 seems to say you should have other BIOS settings to choose from:

  1. Dell Dimension E521 Video Controller Driver Windows 10

This setting specifies which video controller is primary when there is two video controllers availble (PCI, Onboard, Auto, and PEG). This selection only matters when if there are two video controllers present. When Auto is selected, the add-in video controller will be used. NOTE: A PCI Express graphics card will override the integrated video controller

Which setting are you using? In these circumstances, it seems like the right choice would be PCI, if that's available.

Dell Dimension C521 Drivers Download This page contains the list of device drivers for Dell Dimension C521. To download the proper driver, first choose your operating system, then find your device name and click the download button. Jan 07, 2016  Dell dimension E520 desktop, can it be upgraded to windows 10? Page 1 of 2 1 2 Last. Jump to page. (Dell 8900 System) in Drivers and Hardware. Hello, I have a new Dell 8900 series desktop. (64 bit) Apparently has Realtek integrated audio on the Motherboard. Desktop icons position/dimension reset every time I sign-out/sign-in in General.

Controller

It would also be good to verify you are using the latest BIOS level, if you haven't already checked that.

Dell Dimension E521 Video Controller Driver Windows 10

Do not worry about which level of Nvidia driver you are using. Any recent one is good enough and shouldn't have any bearing on this problem.

You may find better suggestions for this via Google rather than here. There's at least one good discussion at https://forums.geforce.com/default/topic/860411/geforce-drivers/this-pc-is-unable-to-run-windows-10/1/.

I've exhausted every solution trying to get this card to work on my PC.
I dual boot Ubuntu 14.04 and Windows XP, and the card works perfectly fine on Ubuntu.
But for some reason, every time I install the recommended drivers from AMD (Catalyst Control Center 14.4,) and restart the system on WinXP, everything works great until the next reboot/shutdown.
Afterwards, I get the 'No AMD driver found or blah blah blah,' message, indicating something wrong.
I've shut off Windows Update, forced Windows to ignore driver signing, cleaned up with Guru and CCleaner and did a fresh install, I even went into the BIOS and changed the primary display to the Radeon card instead of the integrated one; all of these suggestions I've found here on Toms Hardware.
NOTHING has worked so far.
If you need any further information, such as the updates I have installed or other things, I'll provide it.
If anyone can help, It would be very greatly appreciated.