It looks like you have the return value set like this:
<input type="hidden" name="return" id="return" value="http://www.pcsarticles.com/download_center.php">
This should be a valid value for the user to return to once the transaction is complete. Do you have auto return setup in your profile? If not this might be the problem, here is a link to the auto return info:
express_return_summary-outside