What kind of graphics card? If you have a psu powered card and it has taken a dump your system will start to power up and then instantly shut down. Stick the video card in another puter and see if you get the same problem or normal boot. any dead psu powered ati or nvidia card will cause this problem if it has foobared.
Noli Nothis Permittere Te Terere!!
EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.