A couple of months ago I installed a new modem. When I restarted the computer it would not boot up, the monitor just kept flashing up the message "off in 5 seconds", the hard drive light was on permanently, and the DVD-ROM and the CD-RW drive lights kept cycling on and off. I had no error messages or beeps. At this time I could not use the power button or the reset button on the front of the computer, I could only switch off at the wall socket.
I enlisted the help of friend who diagnosed a faulty graphics card, and we fitted an old Ati Rage 11c. The original card was a GeForce 4 mx440, 18 months old. After a couple of false starts the computer started and now works ok as long as it is not interfered with. I did try removing the modem but the problem was still there. I would like to upgrade the graphics card again. I have tried a couple of cards that are known to be good, but encounter the same problem, even refitting the Ati Rage 11c is problematic but once it starts it's ok.