That is "Yellow Light of Death" and it is toast.
Assuming it is an old (and not the new slim one) it means that your board has overheated and needs to be "re-flowed" with a heat gun. It requires opening the entire thing up, taking it completely apart, cleaning off thermal compound from the CPUs, re-flowing the soder with a heat gun, applying new thermal compound, and putting the whole thing together again.
If you have a heat gun, the total cost for this will be $6 and a couple hours of your time (you can buy "Antec Silver Thermal Compound" at an electronics store for about $6).
If you don't have and can't borrow a heat gun from someone, you can buy one for less than $20.
It is also useful to have rubbing alcohol to clean off the old compound from the CPUs.
Here is the guide I used:
http://www.yellowlight.co.uk/documents/YLODfix.pdf
And the youtube series, by the author of the guide (this is Part 1, there are 6 Parts in total):
http://www.youtube.com/user/gilksy1#p/c ... 70SgRDVcBo
Good luck, and if you try this, take your time, follow all the directions, and be careful with the plastic snap-down clips on the ribbon cables.