That can be caused by a fault in either the video card or the monitor; we'll need to narrow thing down a bit. Try the following troubleshooting steps and give us the full and exact details of the results:
* If possible, connect a different (know-to-be-working) monitor. Does the problem still occur?
* Connect the suspect monitor to a different computer. Does the monitor still blink/flash?
* Check the VGA connector(s) at the end(s) of the monitor cable. Do you see any bent/broken pins? Does the monitor cable look severely bent/kinked or damaged in any other way? Try a different cable if possible.
* Boot Windows in to Safe Mode; this forces Windows to use its own generic VGA video driver. Does the problem still occur? (You get to the safe mode boot option by hitting the F8 key as your computer is starting up.)
* Upgrade/uninstall/reinstall your video card's drivers.
* Open the computer's case; make sure your graphics card and othr components are fully and properly seated in their slots, verify that all fans are working, and that the interior of the computer is free of dust/dirt/etc.