How to boot when I get the "do not enter" sign

I am working to help a colleague that gets a "Do Not Enter" sing on boot-up of an iMac that was purchased through the university that we work with. From reading other posts, it was suggested that we re-install the OS to overcome the issue; on the assumption it is created by a corrupt file. This has proven not to work and we still get the same "Prohibited" sign during the start-up process. When Disk Utility is run, the boot sectors appear to fine and do not require specific repairs. Disk permissions were also repaired, but neither process has helped or corrected the problem. I am curious if others have encountered this issue and whether there is an easy work around? I came across one potential cause/solution - a bad SATA cable - however, I am unclear how the recovery disk could boot and run if the IO cable was bad, or even if the HDD was failing and working only intermittently. Any help/solution would be much appreciated. Thanks.