Trying to make E-commerce

I was following a guide to try and add a test product, everything was going fine until I tried the checkout.

I was hit with a HTTP 404 error, and now I keep getting them when I click on pages or posts.

This has happened to my homepage too:

Fatal error: WPSC_Countries::get_countries(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "WPSC_Data_Map" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/content/p3pnexwpnas04_data03/42/2454842/html/wp-content/plugins/wp-e-commerce/wpsc-includes/wpsc-countries.class.php on line 462

Fatal error: WPSC_Countries::_dirty(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "WPSC_Data_Map" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/content/p3pnexwpnas04_data03/42/2454842/html/wp-content/plugins/wp-e-commerce/wpsc-includes/wpsc-countries.class.php on line 1142

I am new and was following a tutorial.

Site is http://asdpecs.co.uk