Insulating the Walls in the Corner of a Room?

I need help insulating the corner of a room:

I used a thermal detector to discover that the corner of the bedroom seems to be missing insulation and is significantly colder than the rest. Living in Colorado and having 2-huge windows in that room does not help. So I would like to fix the problem if I could.

The structure was built in 1984, and I am guessing that something like this occurred (diagram attached to this post) http://www.extremehowto.com/xh/article.asp?article_id=60256

I am betting that the builder did not fill in the gap where the two walls meet forming a 90 degree angle. I would really like to be able to do this myself. Any suggestions? Should I drill some holes directly into the corner where the two walls meet and fill it with some kind of insulation foam? I am a total novice here, so detailed directions would be appreciated. Thank you.

By lisa from Boulder, CO