I am using a Dell lap top with Windows Vista to connect to a NEC VT590 projector. I want to show a PowerPoint presentation. As the computer is booting, the computer monitor is blank and the loading messages are shown on the projector screen. However, as soon as the computer finishes loading, the projector screen goes blank, with the message "computer not connected", and all images are displayed on the computer screen.

Problem is not clear. Are you trying to use remote of the projector to move the slides? And slides from what? A PowerPoint presentation?
If you are using a computer, use the computer to change what is shown. You cannot use the remote of the projector for that.
Provide more information to help you better.

disconnect the cable at the projector end and plug into a monitor to check that the video signal is enabled on the cable. If it is then reconnect to the projector and select a lower resolution on the pc screen setting. See if anything appears on the projector

Here's what the User's Manual says:

Using the Remote Control

To advance PowerPoint slides with the projector's remote, you

must connect the USB cable on the projector's computer cable to

the computer. See page 8 for more information.

Point the remote control at the projection screen or at the front or

rear of the projector (not at the computer). The range for optimum

operation is about 30 feet. If you point the remote at the

projection screen, the distance to the screen and back to the projector

must be less than or equal to 30 feet. For best results, point

the remote directly at the projector.

If you are displaying presentations in Microsoft® PowerPoint®,

you can use the remote's Previous and Next buttons to navigate

through your presentation while in Slide Show mode.

Here's what the User's Manual says:

Using the Remote Control

To advance PowerPoint slides with the projector's remote, you

must connect the USB cable on the projector's computer cable to

the computer. See page 8 for more information.

Point the remote control at the projection screen or at the front or

rear of the projector (not at the computer). The range for optimum

operation is about 30 feet. If you point the remote at the

projection screen, the distance to the screen and back to the projector

must be less than or equal to 30 feet. For best results, point

the remote directly at the projector.

If you are displaying presentations in Microsoft® PowerPoint®,

you can use the remote's Previous and Next buttons to navigate

through your presentation while in Slide Show mode.

It is not the smartboard, lamp or the computer as well know, it's the projector itself. This may not work but you at least need to try press the auto adjust button on the projector and then adjust the clock/phase in the menu. If not you will need to send in to NEC it could by the color wheel. Usually they have a warranty for a couple of years.

• check the cable for any loose connections
• try other cable
• try other computer
  

you can try changing your overlay settings. works with some formats. or you could use the projector as main display (toggle through fn+LCD/CRT)

Red, Green, and Blue signals come from different wires in the VGA cable.  This sometimes happens if a cable simply is not connected fully.  But it will also happen if the cable is damaged.  The damage might be visible in the form of a bent, broken, or missing pin.  Or it might be internal to the cable.
Try another cable.
If that's not it, then the connector on either your computer or on the projector itself might be damaged.
Also try another computer.
Components inside the projector could also fail.  But I'd bet on a cable problem any day.

in layer some add text a' clean filter

I have encountered this same problem once in the office when i was trying to do the same thing.

The solution:
*Assuming you have windows xp*

1) make sure the projector and the laptop are connected (FN+F8), do a right click anywhere in your desktop then go to settings by default Monitor 1 would be your Lattitude D620 and Monitor 2 should be your projector, to make sure of that click on the 'identify' it should show a 1 in your laptop and a 2 on the projector
2) now select monitor 2 and select 'use this device as my primary monitor' check mark, by doing this your videos should play in your monitor but they will be a blank/black screen on the laptop (basically we are reverting the order).

Sincerely
McVerick