As I recall, from my days as a Dell hardware tech, that means that the CPU microcode isn't being executed. There should be a couple of diagnostic LEDs on the case, numbered 1-4, and if any of those are lit, the specific ones will also help pin down the issue. You can compare what you see against the online documentation. I'd just check the codes for the 620, not 520. Not sure they're different, but may as well be safe.
OEM computers, like those from Dell, tend to have very locked down motherboard chipsets. You can't just slap any CPU in them you want and have it work. You get a very narrow range, and that's it.