Encounters an error while trying to reinstall Outpost Firewall Pro

Hi Experts
I want to reinstall Outpost Firewall Pro but I face this error that says a previous version of the same is present in the system and that this must be uninstalled to setup the new version. I had downloaded a new copy of the installer from Agnitum. I also verified that I had been uninstalling the software from a user account with administrative privileges. I am using windows XP Home Service pack 3 and Avast antivirus on my desktop. Can someone guide me to complete the installation properly please? Thanks a lot for your help.

Setup has detected that outpost firewall pro drivers are still
present in system. Please uninstall previous version, reboot, and
run setup again.
