I have a newer HP laptop running Windows Vista (purchased 9/09). Recently, I decided to remove the factory-installed Norton 360 and replace it with something I liked much better (Kaspersky). I've done this numerous times with other computers running the same OS with no problems.
Norton uninstalled cleanly as far as I could tell. HOWEVER, when I installed the new antivirus program, upon re-start, the computer would not boot up fully - I could get to the user icon, but past that the desktop would not open - only a black screen. I was able to run Task Manager, get to the control panel from there, and uninstall Kaspersky, after which the computer promptly returned to normal, the display came back, etc. I then decided to try three other anti-virus programs, one at a time - TrendMicro, Avast, AVG. None of those programs would allow the computer to run either - I got the same black screen each time. And as soon as I uninstalled them, it would return to normal.
I did make sure that I wasn't running more than one anti-virus at the same time - I checked for a clean uninstall on each program. I also made sure to disable Windows Firewall & Windows Defender, as I know these can interfere with Kaspersky somewhat. I also made sure Windows updates were current, and that virus scans were up to date and clean.
Has anyone else ever encountered a problem like this? Could it be that Norton still has files lurking in the system that I can't find, disabling any new anti-virus program?