intel imac keeps rebooting

I have an intel Imac 2.40 GHz "Penryn" Intel Core 2 Duo processor, 250 GB hard drive, 4 GB RAM, 8x multi-format/dual-layer SuperDrive.

I turned it off this afternoon normally without seeing any errors, a couple hours later when i tried turning it back on i only get to the white apple screen with the load wheel and then it reboots.

I've tried holding down shift+d and apple+option+p+r to get any sort of boot but nothing happens.

any ideas?

Thanks in advance

You need to do some trouble shooting with your original install media. Boot the machine with your install DVD (insert DVD, hold down C after you hear the POST chime). After it boots - instead of following through with the install, escape from that and select Utilities, Disk Utilities, from the top menu.

Run the verify and repair procedure and also repair permissions. The repair permissions routine may take a awhile to finish. Let us know.

Regards.