You can't close your subscription/account to the site without being able to log into it (unless the site owner's will do it for you).
You can register again with the site but you'd probably need a separate email address to do so. (Most email providers allow you to set up several 'aliases', with mail to a secondary address going into your regular inbox).
You've not said how you're trying to get a screen shot. The easiest way is to use the Snipping Tool. When you see the error page,click on the Start button, then on All Programs, followed by Accessories and then Snipping Tool. Use your mouse to drag a rectangle around part (or all) of the page. A new window will open, showing what you've just captured. Click on File > Save As and save the file to any convenient location (such as to your Desktop or your Documents folder). Then attach that file to an email to the company.