Dell Dimension 2400 not booting

[gordonj]

My sister has a Dell dimension 2400, the unit worked until a week ago. No components had been added, screen boots with the following message: " A disk read error occurred" "Press Control - Alt - Delete"
selected F12 and went to the utilities, the HDD passes all tests also the DVD/CD passes all tests. Tried to boot from the DVD/CD but the system hangs after saying it is checking system components. Tested the DVD/CD in another computer and it works fine. Also tried the boot CD in another computer and it works. I am getting a volt meter tomorrow to check the power supply. Any other suggestions ??

[gordonj]

To add to the above post: There are four test LED's on the back of the computer and they tell me there is no problem and the computer is functioning properly. Thank you.