Acer Aspire 5100 stuck in boot loop.

I’ve got a problem with my Acer Aspire 5100. It has been fine for years but suddenly developed a problem where it won’t boot at all. It displays the POST splash screen, the screen goes dark then the splash screen screen returns – displaying the “press f2 to enter setup” message. In BIOS it looks fine, the hard disk is displayed and all looks well. It does the same with a different hard disk.If I use a linux bootable CD I can gain access and copy files from the HDD to a USB stick and vice versa, all diagnostics say that all is well but the problem remains. Has anyone seen this before and solved it?