This site has a good code example that can be adjusted to VB
pretty easy:

http://www.qsl.net/kp4md/butrwrth.htm

>I am working on a project in Audiology and am displaying brainwave responses to an audio stimulus, using Visual Basic 5. I want to apply filters to the brainwave data, and am looking for the well-known Butterworth filters. Do you know where I could get such digital filters?? My email address is:
>rtdavey@hotmail.com
>Robert Davey