There are a few useful pages on NI Developer Zone:
http://zone.ni.com/devzone/conceptd.nsf/2d17d611efb58b22862567a9006ffe76/398c0b8f3c5bb436862568c700669c17?OpenDocument
http://zone.ni.com/devzone/conceptd.nsf/2d17d611efb58b22862567a9006ffe76/71cb380bde63face862568c7006d5601?OpenDocument
http://zone.ni.com/devzone/nidzgloss.nsf/webmain/FAE1DC242AD2B42D8625686A00793FE1?OpenDocument&node=DZ59422_US
The link above has a link "digital filter" that has some useful information. There are also LabVIEW Player examples that illustrate many signal processing concepts through code you can run:
http://zone.ni.com/devzone/explprog.nsf/webLabVIEWenabled
If you search on ni.com/zone for digital filter you should find some more documents.
Best of luck,
Kamran - NI