Mac doesn't want to start up

It does not start normally, even if you just have your keyboard/mouse connect to the USB port (and nothing else)?

And that happens with any USB device? If you took out the keyboard/mouse and connected something else, like a printer, the same thing happens?

And you have tried both USB ports?

If you start up with nothing attached, and then connect the keyboard/mouse after startup completes, does it work normally?

Since it's a startup issue, you could try resetting the PMU (power management). The procedure for a slot-loader iMac is shown here.

http://mrjcd.com/junk/PMU.jpg

(As stated, be sure to press the button just ONE TIME between startup attempts.)

If that does not help, you might try getting a self-powered USB hub, if you don't have one already. Then connect all of your USB peripherals to the hub. By self-powered, I mean it has its own power supply. Since the hub is already powered, maybe it won't cause this issue at startup because it would not draw power from the iMac's port, and the connected devices will get power from the hub instead of the iMac's USB port. Current USB 2.0 hubs should work with USB 1.1.

In your System Profiler logs, do you have any error messages reporting a problem with IOUSBFamily?