If you want good results you are going to have to, as celeb_peeper noted, adjust the levels; i.e. brightness, contrast, and gamma. Many programs have built in "brighten" filters that do this for you but depending on the original lighting in the video this can either be to much or to little. The best approach, on average, is to put on two level fx filters, and a secondary (or primary) color correcter fx filter: increase the brightness and contrast of the first level fx filter and increase the gamma of the second level fx filter, then boost the mids with the secondary color corrector (this enables you to balance the lighting so the color is not completely wiped out.) I know Vegas Video gives you these options, and so does Final Cut but WMM is a piece of garbage that is only good for converting the audio levels of whackly encoded avi files to WMA.
Or just post them in the proper section and ask for the video's to be brightned, either I will do it or someone else will.
Cheers