i have a dell xps 420, came home today and the computer was making lou noise, unplugged it, tried to turn it on but it wont boot. heres what happens. the video card fan goes on very loud, monitor is blank, hard drives dont go on, but, all the fans work, cpu fan, fan that blows air into the box, and psu fan works... but the computer wont boot and nothing else works. i reseated the ram, unplugged all the cards, mouse, keyboard, reseated the video card and its the same thing. is my motherboard bad or is there something else?
The fan on your video card probably stopped, and that was the noise you heard it making. Unfortuantely, leaving it on while the fan isn't functioning usually blows the card in a matter of minutes. Since the video card is linked to the BIOS post, all the fans will turn on, but the hard drive won't. If you've got an onboard video card, remove your primary video card and try. It will be a good way of confirming the problem.
You can also get in the bios and disable the fan fail safe. Some won't boot at all if the fans don't come on or it detects a fan error. Could be the power supply. Many big electronic stores will test it for free like Fry's Electronics.