If you have a magnifying glass, I find that this helps to identify some of the characters which are hard to discern.
If you are sure that you are typing the characters exactly as on the label (from inside the retail package) you can contact support (via email, selecting the trial user option)
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve