flashing screen

Did this started now?

yes

Does this happen only on windows start?

yes

Try updating your video drivers or update the BIOS.

You also may want to check your power settings and if it only does it when the computer goes to sleep/hibernate...My computer does this as well. Its eitehr your drivers or power settings. If it continues to do this or if the screen goes blank you may need to replace the video card itself

Hook up an external monitor to the laptop. Does the external monitor also flash as the laptop boots up? If so, then the video adapter is going bad. If the external monitor works fine, then the video adapter in the laptop is fine, the problem lies with the LCD screen, or cable, or inverter, or backlight.

Usually what I do is check all connections, is the LCD cable secure, the wires for the inverter are good, etc.

You can also test buy flexing the screen a little bit at the edges, and opening and closing the lid, to see if the cables are loose, this would possibly make the screen flicker or flash too, as you open and close the lid, the cables would be moving too. Push along the inside bottom of the LCD screen, the bottom bezel....this is where the inverter is located. As you push along the bottom edge, if any cables are loose, it might possibly cause the screen to flash too, as the connection fluctuates.

I do not have a laptop I have an ACER ASPIRE L100 which is a desktop.so no lid to open

try last known good (from safe mode -F8 key during boot)
check the video resolution, has it changed  lately ?

thanks you were correct