Saw someone on here earlier this week with this problem. Turned out his RAM wasn't seated all the way. It would help to know more about your system. I'm going to assume you have 2 sticks of 2 GBs. Take one stick out, can you boot successfully? If so, put it back in and make sure it's seated all the way. Now try to boot.
If that doesn't work, tell us: CPU, RAM, PSU, Motherboard, OS. Basically, the more info you give the easier it will be to help. Is this a new build?
Other things to check:
1. Are you sure your RAM is in the right slots?
2. Download memtest86+ and burn it to a bootable CD. With one stick in your system, run memtest86+ for a few hours or better yet overnight. If it doesn't have any errors, take it out, put the other in, now run memtest86+ on that stick. If it doesn't have nay errors, put both in and run it again.