Boot problems

I was playing Team Fortress 2 and my comp crashed, and when it restarted it froze after the POST, which has happened before, but I was unsure what had been causing it, but after another restart I always booted back up fine. This time though, after I restarted it again, it crashed during the Windows loading screen, and then after the next restart wouldn't boot from my HDD, just show a blank screen with a blinking cursor at the top left. I tried reformatting the hard drive because I can still boot from CD's, but still nothing. I tried using a different hard drive (an IDE one) and disconnecting the SATA one I was using, where I was able to boot from it once and reinstall Windows on it, and after the restart from that I can't get it to pass the POST no matter what I do.

I am feeling fairly certain that it's my motherboard that is causing problems, but if any of you guys have ideas of what else may be wrong or how I could potentially fix it, let me know before I attempt for an RMA.

My specs: AMD Athlon X2 6400+ (3.2GHz); BFG Tech 8600GTS; 2 GB DDR2800 RAM (runs slower than default timings because I thought maybe it was the cause of crashes earlier, so it runs at 600MHz and 5-5-5-15 opposed to 4-4-4-12.); Seagate 320GB 7200 RPM 3GB/s SATA; and some random Dell HDD was the second tested HDD. I'm running XP too.

This is my first time posting here as well, so if there is a better location for this thread, let me know please. I posted here because I suspect that it is my motherboard.

What else would you like to know specs-wise?