Fatal Error - 'Mongo' class not found

The problem was the PHP driver (php_mongo.dll). Apache/WAMP just did not like the mongo-1.1.4-php5.3vc6ts version for some reason. My Windows 7 Professional machine had the mongo-1.2.5-php5.3vc9ts version. That was the only difference between the two machines.

I originally had been using the mongo-1.2.5-php5.3vc9ts version of the driver on the Windows 7 Enterprise machine but changed to the mongo-1.1.4-php5.3vc6tsversion when I was troubleshooting the problem. So the original problem could actually just have been that the database path data\db didn't exist/couldn't be found (and needed to be specified using --dbpath when running/starting mongod.exe).

If this problem exist in php CLI you should add extension=mongo.so to your /etc/php5/cli/php.ini file