dell desktop screen flashing

Does it only flash after loading or attempting to load Windows? or does it flash when you first power it on?
Do you get any beeps when you power it on?

If it only flashes when Windows loads, try running the Dell hardware diagnostics accessed by F12 at Power On. If that passes, my only suggestion is to do a factory reinstall from the Dell PC restore accessed by F8 at power on.

If it flashes when first powered on, that suggests a hardware problem. If it is still in warranty contact Dell support, if it is not, sadly the all in ones are basically a one motherboard with everything built on the board (like a laptop).

One final thing you can try, if this has a port for an external monitor, you could try that to see if its just the monitor that is the problem. Otherwise a new motherboard is likely what is required to fix it (or better yet, if this an older machine it may be time for a new one - sorry).