In rare cases, the .msi installer can fail with the error "The specified account already exists". As far as we can tell, this is not a bug within Canopy's installer but rather with .msi installers in general, where certain registry keys can become corrupted during an install or uninstall.
Microsoft recommends running their fix-up utility and then reattempt the installation. We have found that this has resolved the issue in the few cases we've encountered.
https://support.microsoft.com/en-us/help/17588/fix-problems-that-block-programs-from-being-installed-or-removed
If the Microsoft install cleaner utility is unable to repair the failed install/uninstall, the only remaining option for installing Canopy would be to create a new user account and install Canopy from that user.
It Worked For ME !!! Thanks For The Tip
Microsoft's fix-up found no problems. Still I cannot install msxml4.msi file.
The above-linked article does say that it applies to Windows 10. More feedback welcome.