Monitor switching of at start up. ( Virus? )

[Grumpy]

  Monitor switching of at start up.

Hi my name is Grumpy and I have been ask to try and find a fix for this problem.
This has happened in the last 2 weeks to 2 unrelated computers. So it has to be a virus.
One Computer is running Vista Home and the other is running XP SP3.
What happens?,,
When the computer is switched on the monitor comes on. Then 2 or 3 seconds in the monitor switches off
 ( blacks out ) so you can not do any thing.
You cannot get in to safe mode.
Have changed monitors, same thing happens.
Have taken out the Graphic Card and tried it on the built in card, same thing happens.
Oh yes one is a lap top and the other a desk top.

  Hope someone can give me a fix.
                     
                                               Thank you  Grumpy

[Geek-9pm]

More information needed. Do you mean the monitor will not come back on unless you shut down the system?
You said after 2-3. What? Seconds, Minutes, Hours, Days?
What happens when you turn off the monitor for 10 seconds and then back on?