When you upgrade your localhost’s Apache (aren’t you running one?), and you switch from one PHP package to another, your PEAR might break.
Mine sure did! I got the German Apache/PHP/MySQL distribution package XAMPP from Apachefriends.org, which was quite great, including many interesting things in the package, unlike the more vanilla French distribution EasyPHP, which has served me for a number of months.
So, I urge you to do this little configuration, before you despair:
pear config-show
This will inform you somewhat of your PEAR settings. Then you can:
pear config-set php_dir C://apachefriends/xampp/php/pear
For instance.