Disk Boot Failure

Now I get:

DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER

It won't boot even if I place an Ubuntu DVD in there. My two DVD drives were being recognized before I put it in the power supply which has:

20-pin ATX Main Power Connector
ATX12V Power Connector (cube)
1 4-pin Floppy Drive
Connector
3 4-pin Molex Peripheral Connectors

Please review my installation:

From the power supply the Main Power Connector is connected to the motherboard, 2 4-pin Molex Connectors, that are connected to each other, plug into the two DVD drives. One 4-pin Molex Connector goes to a SATA Y-Adapter which goes to an SATA hard drive and an IDE hard drive.

What am I missing?

Just follow the Guides prepared for the step by step installation process for the hardware peripherals,this will really help you solve your hardware installation process.
http://forums.techarena.in/guides-tutorials/1074207.htm

and following here you will get the Disk boot failure forum discussion:
http://forums.techarena.in/xp-hardware/592334.htm
http://forums.techarena.in/xp-hardware/623288.htm
http://forums.techarena.in/vista-help/757494.htm

I was facing same problem, and the solution for me was connecting my HDD to another PC as slave device , taking all backup and deleting all partitions formatting it. and repartitioning it with windows 98 cd (You can use something else like partition magic)after re-connecting on my pc . Hope this will help.

That PC worked now. It turned out that there was a loose ribbon into the SATA hard drive. The DVD drives still are being recognized.