Don't ask why but.....

Have 184 SQL Server Tables in one database (plus the system tables). Many of these tables have fields which have a dbNull value in them. What needs to happen is all of these need to be replaced with a "", with the exception of the system tables.

Does anyone know a quick way to do this? The thought of going though almost 200 tables and finding the names of all the columns and such seems insane...surely there is a better way??? (I hope I hope)

-Thanks