Ok so after Jeff (@Pye Brook) called in for reinforcements heres what i`d like you to try:
1. Backup your website. Do this first so you have a copy in case something goes wrong.
2. Using FTP go in the wp-content/uploads/wpsc folder and remove the folder called "themes".
After step 2 the list of files in the Store->Presentation should look like the one in the image below:
If they look like that then proceed with step 3.
3. Go to Store -> Presentation page and on the right side box (Advanced theme settings) click the Flush Theme Cache button.
Now retry your Checkout page again.
Your theme was using wpec theme files that belonged to WP e-Commerce version 3.6 (quite old).
Let us know how it goes after that.