Video Looks Squashed/ Add Black frames?

I was posting my first video and I noticed it looked all squashed. I think if I could add the black side frames it wont stretch it so much. I've seen a few videos like this. I believe the website changes the video to payback at 16:9 format (widescreen) and I recorded in a 4:3 format (reg screen.) If I resave it to 16:9 it looks squashed as well. Is there an easy way to pad the video with black space (and keeping the 4:3 format) so it shows up correctly? I use windows movie maker and have XP. Thanks!