Have you tried re-seating your video card and uninstalling all the drivers ??? When you have it pulled out see if you can see any damage to the card like blown capacitors or if it smells like burnt plastic the card might have quit on you.
Also you might try putting in a different card in to it, swop it out with another one in the office and see if on boot up the new card is found. If the new card is found on boot up I say its time for a new card.
i haven't clean my pc since i've bought it.
i really don`t have any idea how to clean the inside of PC.
and its almost 1 year to me.
could this be the reason?
~> but i didn't smell anything when i`m using my pc.
[btw, whenever i saw screen resolution and go in advance settings i always see changing of adapters]
[sometimes, ATI Radeon 2400 HD Pro, Standard VGA Adapter, or Generic PnP-Non Monitor]
just thought it might help.