Adding the following on a new line in your 'includes/config.php' file will disable all plugins:
Code:
define('DISABLE_HOOKS', true);
You can upload fresh copies of all your vB core files to undo any edits you may have performed to any of your PHP files.
There is also a feature in the vB AdminCP under "Maintenance" that is called "Check for Suspect Files"; this will show you any files whose contents don't match what default vB files should contain and also any non-native files in your forum directory.