One of two things... RAM or PSU. I suggest trying to see if it's the RAM first. It's quite possible that your RAM is rated at a voltage higher than 1.8v, which is the default the board sets it at. To make sure you have the right settings, this method has worked for setting your RAM up. Use one stick only, put it in bank 4. Do a proper clear of the BIOS (turn off all power, including PSU switch, unplug, press power button several times to drain capacitors, clear the CMOS using the switch on the back or the jumper (leave the jumper on clear), take out the CMOS battery, let it sit for 20 mins or more, put the battery back in, set the jumper / switch to normal operation, plug it in, switch on the PSU, power it up), when it boots up this time, give it some time. If you have the IP35-E or IP35, you may very likely encounter the "double-boot" issue associated with those boards, if you have the Pro, don't worry about it. If you have the double boot issue, it will boot up, restart itself, and then boot up again, but on the second boot you will get a successful POST. Once the POST comes up, enter the BIOS and set your RAM to the manufacturer's specified voltage and timing (this is very important). Set the timings by hand, don't rely on Auto detect. Once you have set the timings (still with just that one stick in bank 4), go ahead and and save and exit. Shut 'er down, put in your other memory stick and boot it back up.
If all of the above still does not work (and you are sure your RAM is good in the first place... try hooking up a speaker to the motherboard to listen for beep codes if you don't have the LCD on board), you probably are suffering from the Antec / Abit incompatibility problems that have been plaguing people with this board (and possibly the P35 chipset all together).
If you check out the Abit forums (
http://forum.abit-usa.com/), you will see that there is a common problem with Antec power supplies and Abit motherboards, especially with the IP35 series boards. One suggestion is to load the computer with all the devices you will use (hard drives, video card(s), sound card, NICs, etc.) and see if it will start up with a heavy load, sometimes it takes a load to get it started. But more than likely, it is just plain bad luck, and your PSU is likely incompatible with the board. I don't see that you specify which model you have, but if you have the Pro, it will have an LED diagnostic on the board that will tell you where it is hanging up. If you have another PSU lying around, or can obtain one from a friend or something, give it a shot with a different brand. Let me know if any of this helped.
One last thing about the board... update to BIOS 11 as soon as you get up and running, and use the DOS / Command Line flasher instead of the Windows flasher. If you have a 100% reliable and stable system... sure... the Windows flasher *probably* will work, but why risk it eh? I believe the CD Abit gives you is bootable, or you can always use a bootable floppy or USB stick or CD you made yourself. There is a batch file (possibly two). One is called runme.bat I am prett sure. Edit it, and you should see the line that commands the flash to begin, something like awdflash.exe <biosfilename> and then a bunch of switches (ex. things that look like /ty /n4 or something). Add the /wb switch to that line, this will tell it to overwrite the BIOS block that stores some other settings that need to be overwritten. I've heard quite a few people say they have solved stability issues (particularly with RAM) with that option, so it can't hurt, only will ensure you have the best flash possible. Once you are done flashing, do a ceremonial clear of the CMOS, load Optimized Defaults, set RAM voltage, save & exit, reboot. Go back into the BIOS, setup RAM timings and other customized things you had before.
Caz
Edit: Added tip for upgrading BIOS