Video Files are not shown in my compositions and I can't preview them

Hello Adobe community,

since I have a new computer (I think... I can't pinpoint the exact moment this started to happen), I can't use any kind of video files in after effects. I can import them into the library, I can open the footage from the library and play it normally, but once I drag them into a composition, the problem begins:

First of all, the video files are not visible in my work area. I can see text, shapes, even swfs (I work with animate too), but the video files - regardless of the format - are just black. I can hear de audio, I can move them around and trim them in the timeline, but I can't see them. And I can't preview my compositions. It pre-renders 1 or 2 frames, 3 on a good day, but then it stops and the image of the video files remains invisible.

I've tried assigning 12 GB Ram to after effects, I've tried changing all the preview preferences, but the problem remains. I've tried working with only one monitor, re-installing after effects multiple times, updating my graphics card driver and windows directx......

Could anyone help me with this?

Here's some information about my computer:

intel core i7-7700 CPU @ 3.60 GHz 360.GHz
RAM: 16 GB
Windows 7 - 64 Bit
Graphics card: Radeon HD 5450

Thank you so much in advance to anyone who could shed some light!