What is a right approach to install CIS?

Hi. I'd like to switch safely to CIS, since my last attempt ended with both ZoneAlarm, avast! and Windows Security Center corrupted.

Here go my specs:

OS: Windows Vista Home Basic SP2
1GB RAM, 120 GB HD (79GB free), SiS Mirage Graphics 3 (128MB own + 160MB shared)
Current AVs: Windows Defender, avast! Home 4
Current firewall: ZoneAlarm Free 7.1 (which according to Matousec is very poor, that's why I want to change to CIS)

Last attempt, as stated in https://forums.comodo.com/defense_help/secunia_psi_does_not_work-t43193.0.html, ended in Secunia PSI's DB corrupted, and I had to rollback with System Restore and reinstall ZA and avast!.

Am I wrong if I say that I should first deactivate UAC and Defender, reboot twice, uninstall ZA, reboot twice and then trying to install CIS? Oh, and please do notice that I'd like firewall only, since it seemed that Defense+ didn't like PSI. Unless I can finally have D+ work, of course Angel

Thanks in advance!