Windows won't boot - sends me to Windows Boot Manager

MY computer has been around for quite a few years, and a problem like this had never occured. I havn't done anything out of the ordinary lately. It is a desktop computer which came with windows vista originally, but i've installed windows 7, windows 8 and windows 8.1. The problem is that a while ago i returned from school hoping to use my computer and suddenly, as it boots, I saw some strange "American Megatrends" screen i had never seen before. I wondered why that screen would appear, as i had never seen it before. As it dissapeared, the pc continued to boot normally with a few black screens, but suddenly a black screen "Windows Boot Manager" pops up and makes me choose out of two options:

"Choose an operating system to start, or press TAB to select a tool:

The first one says "Ramdisk Settings" and if i choose that, the screen just turns black and the power switch wouldn't even respond.

The second option says "Windows Vista/Windows Server 2008" and when I choose that, another windows boot manager screen comes up which says:

"Windows failed to start. A recent hardware or software change might be the cause. To fix the problem: 

1. Insert your windows installation disc and restart your computer.
2. Choose your language setting and the click "Next".
3. Click "Repair your computer'

If you do not have the disc, contact your system administrator or computer manufacturer for assistance.

File: \Windows\system32\winload.exe

Status: 0xc0000225

Info: The selected entry could not be loaded because the application is missing or corrupt.

If i press continue, the previous screen comes up again. If i press ESC, the screen just goes black.

On the blue screen that says HP Invent (normal booting screen) which has options and F11 is to apply a recovery, the computer won't respond to that F11 key and it continues to boot. So i cannot make a recovery.

All of this is really strange because the time settings on the pc were also changed to 11 / 06 / 2074. (I somehow accessed that screen; the keys on the Hp invent screen won't work anymore) Also, yesterday morning the computer did turn on normally but there was no internet and the antivirus appeared to have been screwed, as it said my subscription had ended and other stuff, and the time settings were on 2069. As soon as i reset the time settings to normal, everything went into place again and everything started working again, I could access webpages and the antivirus fixed up. This is why the time settings i just saw may be relevant to what's going on with my computer, but honestly I have no idea. I did change those time settings but it did not do much. Everything appears to be the same.

Anything I can do if I do not possess the Windows installation disc? Besides, from the threads i've read about similar boot problems, apparently re-installing the OS doesn't even repair this...

Also, i've read about repairing options but in my situation i do not see a single repair option throughout the whole 'boot' process (from power-on till this 'windows boot manager' screen)

Finally, There's also a "tools" option below the 'ramdisk settings' and 'windows vista/windows Server 2008' settings; the tools option reads "Windows Memory Diagnostic" and when i choose it, the same screen reads as above, except the file and status are different:

File: \boot\memtest.exe

Status: 0xc000000e

Info: The selected entry could not be loaded because the application is missing or corrupt.

I have removed my external hard disk drive and attempted booting, but everything remains the same. This is the first time something like this has ever happened to me, so I'm new to this and I might be missing out some important information which I should be listing here. If such is the case, please answer with any extra information you might need to help me out!!!!

Thanks in advance, any attempt of resolving this will be greatly appreciated.