[eluser]mattthehoople[/eluser]
If you change the db log in details to be incorrect you might get a "cannot connect to your database with the supplied details" type error. If so CI is fine and it may well be a problem with your mysql installation. Check your apache logs after a restart for stuff like...
PHP Warning: PHP Startup: Unable to load dynamic library './php_mysql.dll' - The specified module could not be found.\r\n in Unknown on line 0
It's a problem I'm having with xampp on an old machine at the minute... If you get it fixed, let me know how...