Just put your head on the desktop!
Serioously: you did not specify what OS you are on, but I assume the mainstream Windows - in that case you have the tool already on your PC -open Windows Movie Maker in your Accessories, go to Tools >> Video Effects >> Turn 90° (or 270°)
If it is not what you search, then just enter "rotate video 90 degrees" into Google and you'll get dozens of different alternative solutions.
