Instalation or extraction problems

Hi… I need your help… I downloaded the avast4! free Home edition and I can not install it because it appears a message that sais: “Not enough user rights to continue.” and doesn’t let me install the antivirus or even extract it.

What OS are you using? is it up to date?

If XP, what rights does your user account have?
Are you trying to install avast as the Administrator or a user account with administrator rights?