STEP1: USE the uninstall tool for Bitdefender 2013 from the link below:

http://www.bitdefender.com/files/KnowledgeBase/file/BD2013_Uninstall_Tool.exe

Step2:  RUN a CCleaner tool to remove all the old registry files, download the CCleaner from the link below:

http://www.filehippo.com/download_ccleaner/

Step3. Restart the PC and re-try to install Bitdefender 2011/2012 once more.