Cannot boot into Windows 7 after installing Windows 10 Tech Preview

So recently, I've installed Windows 10 on a separate partition on my 500 GB hard disk that's about 40 GB, and it works perfectly. Except that I can't boot back into Windows 7.

All of my files are still accessible and working, I just can't actually boot into the OS. When I attempt to through the Windows 10 boot screen, first it reboots, then it goes to a black screen with 8 short lines of bright blue pixels on the top left. After a couple of moments, multi-colored pixels start appearing and changing all across the top couple of lines of pixels. This happens in the course of about a minute or so. I usually give up and turn it off cause I notice there is obviously something wrong.

Is there anything I could do to fix this? I've looked everywhere to see if anyone had this problem, but I couldn't seem to find anything.

Thanks in advance!

PS: This is my first time using this forum, so excuse my nooby mistakes.

This is a known problem, you`ll find answers on how to repair it at Tenforums.

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