As deanimator said, you can remove the audio stream, try to run it through a filter, then recombine the audio and video. You can use Virtualdub to extract the audio to a .wav file. Finding a good audio program that can do the filtering for free is the problem. Audacity is free, but from what I’ve read, this type of filtering is not one of its strong points. Do a Google search on “remove hiss with Audacity”.
You could also try the free trial of Goldwave. It is supposed to be fully functional for the length of the trial.