Help!! Error loading operating system

There is a problem with my old PC (Windows 7 x32, Intel duo core processor, 2.5 GB RAM) which was custom or assembled, it says 'Error loading operating system' and I have been trying to fix it for hours..
Earlier when this happened (A lot of times), I fixed it from the Repair option when I press F8 contineously, but when i keep on pressing it, nothing happens (System Repair Blackscreen does not come).
It happened when I pressed the restart button in my CPU when my PC was stuck for a long time, instead of waiting and manually restarting.
The biggest problem is that I LOST my Windows 7 CD. All I have now is just a USB with backup files.
When I put on the USB while booting, it says 'BOOTMGR is missing, Press Ctrl+Alt+Del to restart' but it shows the same thing when I restart.
When I pressed F10, It showed the Boot Option Menu, it had 3 options:
• SATA: Optiarc DVD RW AD-72203
• Realtek Boot Agent
• MBIC SS MMoser Baee Disk PMAP (That one is my USB)
These 3 didn't work either....
So, Please help me to find a way to fix it.
Ask me if you want any more information.
Thanks...

~Rex123
Thanks now its solved :)

Do you have the product key printed on the computer chassis somewhere? If you do, you can use a friend's exactly-the-same Windows OS DVD, and make sure you enter product key off of your computer chassis correctly.

Rex123 you have 3 new threads on this same problem on the same computer.
That is a NO NO.

One thread, one problem, one computer. Other wise members trying to help you get lost.

Please read the forum rules.

https://www.sevenforums.com/misc.php?do=showrules

2) No useless posts, e.g., thread bumping, useless one liners, junk posting to increase post count, and identical multi-postings in different forums.