I recently tried to BOOT my computer, but instead the monitor showed a check signal sign with the three colors.  After reading I figured out it was the monitor's self test. 
I've switched systems and my monitor works fine, so I figure it's not the monitor or cable.  I reinserted the video card and it didn't work.  I bought another video card (an upgrade for me anyway) and it still doesn't work. 
Is it the motherboard? Powersupply?
I don't know if this helps but when I boot the computer all the fans start to run.

I've had this computer for two years now.
CPU : AMD 64 x2 4200 Manchester 2.2ghz
MotherBoard : ASUS A8N-E Socket 939
Old Vid Card: EVGA 256-P2 N564-AX GeForce 7900GT 256Mb PCI Express x16
NEW Vid Card: NVIDIA GeForce 8400 GS 512MB PCI Express x16
Monitor : Samsung SyncMaster 204b

Note: When I push the adjust button on the monitor it switches back from analog to digital a bunch of times then the screen turns black.Were you able to try the video cards in a different computer?  (I know that's not always easy to do.)  I shouldn't wonder if the PCI Express slot went out.  Not likely, but possible, I guess.

Having spouted all that, I must ask if you know if the most updated drivers for the video cards were installed?No luck with switching the video cards out, my other SYSTEM has a different slot type.
When I inserted the new vid card I didn't UPDATE the drivers, cuz I figured it would at least show up on the monitor.
Even THOUGH the drivers arn't updated, shouldn't i see something other than the self test on the monitor?Agreed.  You should get basic video function.  I was just looking for another way to test the cards themselves, while I stated the idea that the express slot on the system board is/went bad.  Being able to slot the cards into another computer would help. 

It occurs to me to ask if your computer has on-board video you might try as an option?Depending on the card you might get a VGA output or no output at all without drivers.
Didn't the card come with a driver CD ? ?

Don't rely on Windows to update drivers...especially video drivers.i think your vga slot got a problem.you should try your vga card to other pc if it works fine then its your board got a problem