I have not grokked your error at the moment, but I CAN answer part of your question.PEAR is a framework and distribution system for reusable PHP components.
PEAR is to PHP, what CPAN is to Perl. They are both standardized repositories for add-in Modules/packages. Modules are generalized extensions to the core languages, usually engineered for a specific task. Things like authentication methods, mail handlers, uploaders, and security enhancements all fit the bill.
Do you "need it?" - Only if your PHP code requires installation of extensions (dependencies). http://pear.php.net/Hot off the Press
PEAR channels on google code currently broken
It 'may' be that your error is casued by this PEAR issue - although mine is not broken when I access the page. The default install does point to googlecode.D:\Program Files\xampp\php\PEAR\PEAR\Registry.php
I don't know the version of Windows you are on, but you might consider installing XAMPP in the root of one of your named drives. i.e. C:\XAMPP or D:\XAMPP. I have seen topics where this fixed some oddball issues.
You also might just try replacing this one file with the origianl extracted for your download.
I also have a question, when you did this upgrade/replacement - did you do a clean install, and then relocate your projects, or did you 'install over' your old version?