My monitor a compaq FP5315 has a white box

It means that the PC is putting out a frequency that is not readable by the monitor. Basically comes down to the hertz that the computer is set to.

To fix this:

What you need to do is go into the boot menu by pressing and holding the F8 key just after the text of the boot process begins appearing on your screen until a list of options comes up on the screen. One of those will be to enable VGA mode. This will set Windows back to using the basic 800 x 600 resolution. Highlight that option (using the arrow keys) and then press Enter. Your screen should work just fine now.

From this point you can choose a resolution that fits into the range of the monitor. Be sure to "Test" the settings before applying them to be sure you don't run into the same problem again