So did you change the name of the dll?
Ways to change the MD5:
-Use a crpyter on it or binder (If you use someone elses public program Im sure it will bind their virus to it.)
-Compile the dll from its original source code change the method names and other variables.
-Hex editor, edit any plain text do not add or subtract any characters just replace
Why do you need to change the MD5 anyways?