Getting tilted video from a helmet mount POV cam seem to be a common thing. I get the same same thing even after correcting the angle of the mount. I am guessing the weight of the camera and the force of gravity is more powerful than my neck muscle.

I tried using Window movie maker and gave up with it. I chosed to use Sony Vega HD Platinum. It has many feature for editing video including fixing a tilted clip. However you will lose some of the original frame because it will crop the frame after the tilt is corrected. This is something that cannot be avoided. Imagine taking a frame and tilt it clockwise a few degree and than crop it back into a rectangular shape that's level. You will lose some of the corner in that process.

The software can be had anywhere from $45 for the older version to $80 for a newer versoin. It is worth the investment if you are planning to do many video editing.