Imac front led flashes on startup

I have an Imac 20", Intel based that won't startup from the first time. The only thing that happens is a slow flashing white LED on the front. I've been back to my reseller because they thought it had something to do with the memory they added. But as usual the phenomenon didn't accure at the shop. Back at the office the first times everything was ok. But the day after the flashing led appeared again.
When I detach all the USB it startsup normaly (only keyboard and ups). Even when I reconnect the keyboard during startup it halts.
Does anyone has encountered the same problems?
strouvenbaAsked:
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Eoin OSullivanConsultantCommented:
Flashing or glowing LED on the front ususally implies that it is in standby mode.

Insert the System DVD and reboot (hold down the power button on the back until it restarts) as soon as it starts hold down the C key to force it to boot from the System DVD.

On the first screen of the Installation Menu, go to the Apple menu and select Disk Utility .. run repair permissions and verify the disk.

Shutdown and restart .. see does this help.

If not go to the next stage .. Reboot and hold down the Apple and S keys .. this starts in single user mode. You'll

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