Get memtest86+. http://www.memtest.org/
Make sure the RAM is ok. Run the test for a good solid hour or two. If you get a few errors in that long of a time, the RAM should be fine. But if you start getting many errors within the first few tests, then you got a problem. However, I have seen these problems caused by bad RAM or bad mobo. So if you have 2 sticks of RAM, try one at a time to see if you can get the system working. If neither works, then that doesn't help narrow down if it's mobo or RAM related issues. By then I would try the RAM in a friends system and see if it works fine. If they work, bad mobo. If not, most likely bad RAM.