Database connection error (3): Could not connect to database’,

I upgrade my xampp to 1.8 from 1.7 by uninstalling 1.7 but did not uninstall the htdocs and mysql data folder. But my problem now is that my site are not displaying, my joomla 2.5 sites give: Database connection error (3): Could not connect to database, while joomla 1.5 sites give errors such as:

Strict Standards: Non-static method JLoader::import() should not be called statically in C:\xampp\htdocs\osu\libraries\joomla\import.php on line 29

Strict Standards: Non-static method JLoader::register() should not be called statically in C:\xampp\htdocs\osu\libraries\loader.php on line 71

Strict Standards: Non-static method JLoader::import() should not be called statically in C:\xampp\htdocs\osu\libraries\joomla\import.php on line 32

Strict Standards: Non-static method JLoader::register() should not be called statically in C:\xampp\htdocs\osu\libraries\loader.php on line 71

Strict Standards: Non-static method JLoader::load() should not be called statically in C:\xampp\htdocs\osu\libraries\loader.php on line 161

Strict Standards: Non-static method JLoader::register() should not be called statically in C:\xampp\htdocs\osu\libraries\loader.php on line 138

Strict Standards: Non-static method JRequest::clean() should not be called statically in C:\xampp\htdocs\osu\libraries\joomla\import.php on line 33

Strict Standards: Non-static method JRequest::_cleanArray() should not be called statically in C:\xampp\htdocs\osu\libraries\joomla\environment\request.php on line 463

Strict Standards: Non-static method JRequest::_cleanArray() should not be called statically in C:\xampp\htdocs\osu\libraries\joomla\environment\request.php on line 464

Strict Standards: Non-static method JRequest::_cleanArray() should not be called statically in C:\xampp\htdocs\osu\libraries\joomla\environment\request.php on line 465

Strict Standards: Non-static method JRequest::_cleanArray() should not be called statically in C:\xampp\htdocs\osu\libraries\joomla\environment\request.php on line 466

Strict Standards: Non-static method JRequest::_cleanArray() should not be called statically in C:\xampp\htdocs\osu\libraries\joomla\environment\request.php on line 467

Strict Standards: Non-static method JRequest::_cleanArray() should not be called statically in C:\xampp\htdocs\osu\libraries\joomla\environment\request.php on line 468

Warning: Cannot modify header information – headers already sent by (output started at C:\xampp\htdocs\osu\libraries\joomla\import.php:46) in C:\xampp\htdocs\osu\libraries\joomla\factory.php on line 566
Database Error: Unable to connect to the database:Could not connect to database

Please what was my error. Is there a special way to have backed up database.
I am now running xampp 1.8.1 on windows 7. Upgraded from xampp 1.7.0