This is a known compatibility issue between the browser plug-ins for Acrobat 6 (Full) and Acrobat 7 (Reader). When both programs are installed on the same machine, and automatic updates are applied, registration for the browser plug-in is corrupted.
This problem affects any application using the Acrobat browser plug-in, including Internet Explorer and other web browsers.
The short-term solution is to uninstall and reinstall Acrobat 7. It is recommended you download the latest version from Adobe to prevent the problem from recurring when the next update is installed.
The long-term solution is to upgrade to Acrobat 7 Standard or Pro and uninstall Acrobat 6.