An Operating System Wasn't Found. (Been trying for ages!)

Hi there! I have a Samsung RV515, which I had running Windows 8, and I decided to go back to Windows 7. When I boot up my machine I get the following error;

"An operating system wasn't found. Try disconnecting any drives that don't contain an operating system."

I've read various posts of other people with the same issue, and I'm actually quite savvy with computers myself - but this has me baffled.

In my BIOS, I have my priorities set to;
1~ USB HDD (which is a 250GB HDD from an old laptop, with W7 on it)
2~ SATA DVD Drive (which currently has a Windows 7 Installation Disk inside)
3 ~ SATA 500Gb HDD (the one on the inside of the laptop)

I've tried Manual Boot Override, both on the USB HDD & the DVD Drive, but I just get sent back to the error. No matter what I seem to do, I cannot get to the repair screen which numerous other posts have claimed I should if following the steps set out for me.

I'm confused as hell, and I've got some really urgent work to get done that I can't do (I'm an amateur Games Composer working on a project at the moment) so I'm desperate to get my machine working again.

What I did was install W7 onto an empty partition on my hard drive, then whenever I restarted my computer, I had the option to use either Windows 7 or 8, but the default became Windows 7.

I kept both for a few weeks, and then decided to remove Windows 8 altogether for now, because a lot of the software I use at the moment isn't compatible even when running in compatibility mode for Windows 7.

To do this, I went into the Create an Manage Hard Drive Partitions option in Settings via the Start Menu (after searching Partition), and I erased the Windows 8 Partition, which left me with my D:/ Partition for all my files etc, at approx 200Gb, my Windows 7 Partition at approx. 230Gb & "Free Space" at 64Gb, where Windows 8 used to be.

It was working absolutely fine last night, then when I went to turn on my laptop today, I got the above error.

Basically, from what I can gather, I've not selected the Windows 7 Partition as the Default Partition, so instead, what my Laptop is doing is trying to boot off the empty Partition.

I've tried using my Windows 7 Disk, and now also an Ubuntu disk just to get something installed on the empty space, but as soon as I choose to boot from the DVDs (and even the old hard drive from my old Laptop which has W7 on it), it sends me straight to this error screen again.

Sorry, I'm confused. How do I use this program if I can't get the laptop to actually boot up into Windows 7? I won't be able to install the program, is there anything that can intercept the boot process on the laptop that's currently not booting up?

"I've tried Manual Boot Override, both on the USB HDD & the DVD Drive, but I just get sent back to the error."

Also, if I press ESC when the Samsung screen appears, I get a wee blue box that pops up asking me to select a boot medium, it also gives me the option to go into my BIOS, and once in there I've tried choosing the DVD & the USB HDD from there as well, but to no avail.

The Hard Drive is definitely working though, so I know it's not a hardware problem.

I'm contemplating opening up the Laptop case and removing the Hard Drive, then plugging it into this laptop (my parent's cruddy machine) as an external at least to rip everything off and then clean the whole thing, but I don't really fancy the risk that I break something when doing so.