Internet Explorer has been known to cause issues during the checkout process for a number of reasons. The checkout system that our site uses is called "Ubercart", and requires a secure checkout environment when used in conjunction with Authorize.net.
Because of this, some server requirements are necessary to make for a secure checkout process. These include the ability to access your browser session instance and cookies.
Some ways to circumvent these issues, is to turn down the security settings on your browser, or to install another browser.