i boot up my system, i get a post beep (yay) but then about a second later, my system shuts down, the case just turns off, but the motherboard still has a green LED, i checked and i have the power and reset plugged in right.
Please provide your complete system specs, including the brand and model (not just wattage) of your PSU.
One quick and dirty possibility is that the power switch is shorting, causing the PC to shut off four-five seconds after it is turned on. To check for this, unplug the power switch, and use something like a screwdriver or key to briefly short the power switch pins. If the PC stays on, you've found your problem. If not, it is most likely IMHO the motherboard or PSU. There could be a problem with the Power Sense line, needed to keep the PSU on, or the PSU is defective. Did you replace or upgrade anything recently, or is this an all-new build?
Your PSU should be of sufficient capacity for your build, but you may have "lucked out" and gotten a bad one. Do you have another PSU you can test in this build?
The other possibility is that the motherboard isn't properly keeping the PowerOK line active. Check in your BIOS and [temporarily] disable any ESD protection, if present. If that fixes it, make sure your PC is plugged into a properly grounded outlet.
Hmmm, as many video cards as come with those, a local PC Shop (or even a reasonably friendly sales or tech droid at an office supply or Best Buy) may have one they'll just give you.
The HDMI not working may be a driver problem. You might let the system boot up. If you see drive flickering, Windows may be installing drivers. Let it do that for at least ten minutes or so. If you don't get a display, a shutdown may be needed. Press the power button briefly and see if that causes the machine to shut down. If it doesn't, you'll need to hold the button in for 4-5 seconds to shut it off. Power back on, and see if drivers loaded that give you a display.