To get a more accurate reading on your available HD space:
Then check again.
Follow the suggestions here as well: http://docs.info.apple.com/article.html?path=Mac/10.7/en/mchlp1235.html
Try using a storage allocation tool to see where the files are that are taking up the space:
Use caution when removing files. If you don't know what a file is, don't remove it. Make note of where that file is stored using one of the tools above and report back with the location.