Download and install this windows installer version from here
Please try this log in as an administrator by rebooting your pc press several times on the F8 button before the windows loading screen now select safe mode choose for administrator. Go to start,now navigate to my computer ,right click on the drive letter of your hard disk where windows and your software is installed on click on the sharing tab and select share this folder. Click Permissions, and select Everyone in Group Or User Names (I mean all user accounts that are currencly on your computer) Select Allow next to Full Control in Permissions, and then click OK.Reboot your pc.
First log on with your administrator account after that press the windows button together with r now type :
cmd press enter.
Now type this:
c: (if that is the partition where you have windows or your software installed on) press enter
cacls "c:\" /T /E /C /G administrators:F press enter. Don't forget enter spaces between /T , /E , /C , and /G.
exit press enter.
Install windows installer uninstall BitDefender once again as you did before reboot your pc. After that reboot again. Try to install again. If you still have problems it might be an infection which is preventing the installation.