Magento – Fatal error: require_once(): Failed opening required ‘D:\xampp\htdocs\magento\errors\report.php’

I think I fixed the issue by updating Magento to 1.8.0.1 and editing

app/code/core/Mage/Catalog/Block/Navigation.php line 254

from

$childrenCount = $children->count();

to

$childrenCount = count($children);

I can access the main page without any error message now!

Wrong username and password or invalid access rights to you MySQL server.

Check app/etc/local.xml and verify your credentials.

If you need to reset you MySQL password you have to access your MySQL shell and use the following command (replace with correct information):

GRANT ALL PRIVILEGES ON yourdbname.* TO 'pctekcom_magento'@'127.0.0.1' IDENTIFIED BY 'yourpassword';

NOTE: MySQL username and password, NOT Magento username and password.

If you have phpMyAdmin you can use the "users" function.

Also try using localhost instead of 127.0.0.1 in your app/etc/local.xml, sometimes it fails the reverse lookup for localhost.