Possible something goofy with the previous install that MCPR removed?
Anti-virus software components insert themselves deep into the operating systems core where they install kernel mode drivers
that load at boot-up and create files/folders/registry entries in various locations. It is not uncommon for some anti-virus programs to not completely uninstall itself using the usual method of Programs and Features
or Add/Remove Programs
in Control Panel due to the presence of related services and processes running in the background.
Most anti-virus vendors provide clean-up utilities
or removal tools
on their web sites to remove remnants left behind after uninstalling, or for a failed uninstall or failed reinstall. It's always best to download directly from the vendor's site to ensure you are using the most current version of the uninstall utility as it is not uncommon for third-party hosting sites to have outdated versions which may not work properly.