I somehow found out how to uninstall.
When trying to re-install, there is an option to "Remove all previously installed components" which I was earlier afraid to do, as there is an added caution that "In addition to removing all software installed, this will remove all user-defined Transaction Server Packages and IIS/PWS configuration backups."
I did click "Remove All" to uninstall previously installed components and re-install but ended up with the same unsuccessful installation messages.
The first to appear on the screen was: "Microsoft Transaction Server Setup - An unknown error occurred while making MTS specific changes to the system registry. Error Code = 0x80004005." After clicking OK to clear this error message box, then came the second one that read: "Setup of 'Transaction Server Core Components failed'. The specific error code is 0x3f9e670. Setup will continue but the component may not function properly."
And true, I can no longer run my Perl/CGI and PHP scripts.