I assume S11 is the last full version and that S12 was purchased as an upgrade. If my memory goes back that far, you do not have to install S11 but only either enter the S11 serial number or pop the S11 disc in the drive as proof that you have S11. However you will not have to go through all that if you leave the 'Pixie' folder intact when deleting the program (it will be left by default).
You could uninstall all of S14 and clean the registry etc by using the Pinnacle regdelete. On the other hand, maybe I can save you some pain and suffering if you were to explain in a few words of not less than 6 pages why you feel it nescessary to deleye all of S12. Reinstalling is not always a panacea to curing a problem.