If the nut is too low, you MAY wish to insert guitar repair glue into the nut string grooves. This will raise the height of the grooves and end the buzzing, this is the "quickfix" method. One caveat however: spread very carefully and cover all other parts of the guitar against potential damage spread by the glue (people are scared to try this on certain instruments because of the potential of spreading this glue into unwanted areas). If you have a rare guitar you might not want to try this solution, but it usually works reguardless, unless there are other issues within the neck to contend with besides the problems associated with the nut.

Dewey