There is no download for a Mac OS X installation disc. You should connect any USB keyboard while trouble-shooting this problem.
it shows a fold icon in the middle and that's all.
That means the iMac cannot find a bootable system. That often (but not always) means there is a problem with the internal hard drive.
You can try a few things. First, reset PRAM
http://support.apple.com/kb/HT1379
It should work with a Windows PC keyboard, using Windows and Alt, instead of Command and Option.
If that does not help, use this procedure to reset the SMU (power management)
http://support.apple.com/kb/HT1767
When you do this, if you have been using a "power strip," try connecting it directly to a wall outlet by itself. Disconnect everything, and initially power it up with only the power cord connected. If it starts up normally, connect only keyboard and mouse. Use it that way for a while to see if any problems come up.
If the problem still recurs, you'll need to have a Mac OS X installation disc (for the currently installed version of Mac OS X) to do additional trouble-shooting on the hard drive.