Problem starting Windows 7

I really am perplexed. My computer was working just fine until about a week ago when I got a message in a red box that read "Secure Boot Violation. the System found unauthorised changes on the firmware, operating system or UEFI drivers. Press [OK] to run the next boot device, or enter directly to BIOS setup if there are no other boot devices installed. Got to BIOS Setup > Advanced > Boot and change the current boot device into other secured boot devices" As far as I was aware, no changes had intentionally been made before this notification appeared. I jiggled with the BIOS settings as suggested on other threads on this forum, with no success. Trying to start in Safe Mode led to a dead end, and eventually I took it back to the people that made the computer for me. They said the hard drive was damaged and they replaced it. So last night I reinstalled Windows 7 from my disc, but on reboot I am faced with the same problem! So i think the hard drive was not the real problem at all. Windows 7 is installed on the hard drive (I can see it there when I tried to do a repair) but it will not open. The install seemed to go smoothly but I could not get it to "find" my broadband connection so I went to reboot it but am now stuck back in the same loop of "start Windows Normally" or "Start Windows Repair" and whichever one I click on, it loops back to the same screen. Any suggestions please?

Thanks for your help. These were the changes I originally attempted before I had the hard drive replaced. They didn't work then for me. I've tried them again now but still no joy. In the Win 7 install process I get to "Where do you want to install Windows?" page, and it gives me two options Disk 0 Partition 1 and Disk 0 Partition 2. But then it says Windows cannot be installed to either of these, because the selected disk has an "MBR partition table. On EFI systems, Windows can only be installed to GPT disks." i have no idea what any of that means.

OK, searching on this forum I have found a fix for that particular issue (thanks Munchbot) so am trying again with the installation and see where I get to this time.

Yes indeed I did. I am having to reinstall as the HDD has been replaced. After much fudging and fiddling, suddenly last night the installation all fell in to place and worked like a charm. I was gob smacked but very thankful. Then this morning, when I booted the computer up, I get the message "there is no bootable drive". Seriously. So I think this is a mixture of hardware and software issues. Thanks for your assistance

I'm too scared to look in case the whole deck of cards comes tumbling down...... Nah, give me a moment,I'll reboot and have a look.

No...neither did I!