How to determine a very low frequency signal? I have a noisy signal with frequency between 1/3 and 1/5 of the sampling window.

DFT has a very low resolution for low frequency components, what is the best way to find the frequence?
Check out this article by Eric Jacobsen and Peter Kootsookos:
http://www.ingelec.uns.edu.ar/pds2803/Materiales/Articulos/AnalisisFrecuencial/04205098.pdf

I had some success using this method for doing "sub-bin" frequency estimation when only one sinusoid is present.