For now the easiest would be to remove PHP5 and restore PHP4 (IF YOU HAVE A WORKING BACKUP)
If not then hopefully you have the PHP.INI file that you can restore, look at and configure.
If not then basically you need to get PHP running again and then look at your UBB site issues (if any, once PHP is working hopefully there won't be any issues).
Try taking a look here first and getting PHP working properly again on IIS
PHPBuilder - IIS6/PHP Setup FAQ Do Not copy PHP.INI to the Windows folder as some suggest, fix the path and set an environment variable called PHPRC and set to the location of your PHP installation.
Don't forget that IIS needs rights to read PHP/MySQL as it's the one doing the work.
