It took me more than a week to track this down, but after installing the most recent Adobe Reader (9.3.4) my mouse wheel became disabled and in the mouse control panel applet, the Wheel tab is not visible. It takes more than a simple 'Restart' for the problem to become apparent (and to be resolved). It takes a full shutdown for the problem to appear after the upgrade and to be resolved after an uninstall of Reader. And Adobe only provides the most recent version on its web site so I can't roll back to an older version, so for now I'm unable to read PDFs. I'm running 32-bit Win 7 Ultimate on an HP Pavilion Media Center TV m7640n Desktop PC with an AMD Dual Core Processor. I had also noticed the problem on my T43 Thinkpad but a subsequent update of the Lenovo UltaNav software apparently resolved it on that machine.