If you system still will not install CIS 8 after uninstalling previous versions using Chiron's method, including running the Geekbuddy tool, you could try the following if you know what you are doing with the registry editor, and file deletion in system directories and know how to do a system restore in safe mode if needed. Please note these are experimental steps based on my personal experience and are not tried and tested. Please note these steps are carried out at your own risk, and do not carry any guarantee that they will work for you and are not approved by Comodo in any way.
These steps may not help if your problem relates to installation on non-unicode system. But probably worth trying anyway.
0. Make a system restore point
1. Boot to safe mode. (Hold F8 down during the first part of the boot cycle on most computers)
2. Run options 1 and 2 of the Geekbuddy tool again
3. Make sure all the Comodo Internet Security files in the following directories have been deleted
Some of these directories will not be present on 32 bit machines. To identify the correct files, sort by company, and check the detail tab in the properties of each Comodo file to see if it is part of Comodo Internet Security.
4. Open Control Panel ~ Admin tools ~ Device manager and switch on view ~ Show hidden devices
5. Find Non-PNP driver Comodo Helper Service and Comodo Firewall Driver right click on them and and uninstall them
6. Go to start ~ Run ~ Regedit
7. Navigate to the following keys and delete or rename them:https://copy.com/lS2HSqrzWDEH/2014-12-14_09-14-51.png
(On 32 bit machines the path ending 'guard64.dll' will end instead in 'guard32.dll')
8. Shutdown and reboot to normal mode
9. If you system experiences boot problems after performing these instructions try system restore first in normal mode, or if not in safe mode.