Program has a "light" version  - Biorhythms for Windows. 

Options that are in Pro version but not in "light":
  Finding of Good and Bad days
  Finding of days, similar to selected ones
  Adjustable duration of biorhythmic cycles
  Lunar phases
  Secondary Biorhythms
  I-Ching Biorhythms

You can always upgrade Biorhythms for Windows to Biorhythms for Windows Pro, paying only price difference between these programs. 
When upgrading, all data you have entered into "light" version will remain intact.