If you use Internet Explorer 7 as your browser, attempts to load pdf files
may result in error messages. The problem appears to be an unforeseen
interaction deep within the new security features of IE7. Try this:

• Open IE7.
• Drop down "Tools" and select "Internet Options."
• Click on the "Advanced" tab (far right).
• Scroll down to the Security section (the last one) Find the toggle box labeled: Allow active content to run in files on My Computer.
• If the box is already checked, click on it to uncheck it.
• Then click Apply and then click OK.
• Restart your computer.
• Note: If the box is NOT already checked, this method will not work.

(Thanks to evolutionary-ecology.com.)