Trouble installing Internet Security 2012 on Mac

Am unable to install Internet Security 2012 (for the Mac) on a Mac OS10.6 computer.  When I try to install, I get the following message: " Macintosh system error -1409".   I followed the on-line advice to uninstall all anti-virus software, but it did not solve the problem.  Any help would be appreciated, since now I have no virus protection.