I tried to uninstall Avast Internet Security as I was unable to use its firewall. ( By the way, I use Windows 7) After having tried unsuccessfully to get the firewall to work I was advised to uninstall the current version, and re-install Avast Internet Security again. However, it won’t let me re-install because it says that there still is a corrupted Avast Internet Security software on my PC. Would someone be able to help me please. I am really quite concerned as I am not that computer literate and worried about leaving my computer exposed.
I would greatly appreciate some help with this. 
www.avast.com > support > downloads > uninstall utility
and you have of course removed any previous used AV using that vendors removal tool
http://singularlabs.com/uninstallers/security-software/
Try a clean install…
Download and save the avast uninstall tool.
Run the uninstaller. You will be directed to reboot into safe mode…click “yes”
Reboot
Re-install Avast Internet Security
Reboot and insert license.
You may need to disable the Windows Firewall.
This is the tool you require http://files.avast.com/iavs9x/avastclear.exe
It runs in SafeMode. Download, and run it , then try a new install.