Windows program for watching mp4 frame-by-frame or slowmo

I have Sony Vegas but my CPU is really bad and scrubbing through a video, a lot of frames are missed. Not to mention the quality is bad. I need to view an mp4 frame-by-frame and/or in super slowmo, in full 1080p resolution, without any frame-skipping or stuttering. What (free) software should I use for this?