Most exe files wont open

Heeelp, am running Win Xp Pro, my exe files on the computer are failing to open,the following error messege is displayed when i try to open any file “Attempt to Access Invalid address” i have run the AVG updated antivirus software, & it has removed many trojan horses includung the win32 blaster virus & i again ran the fixblast tool, but if i try to install Norton antivirus, it will install but wont open after install. Cant afford to format my hard drive