Symptom: You receive the following error message after clicking a link in Outlook: This operation has been cancelled due to restrictions in effect on this computer.
This seems to be caused by uninstalling Firefox (such as I did, when I uninstalled Firefox 3 beta) or whatever your default browser happens to be.
Fix: Go to Control Panel, Add Remove Programs. Then on the left, choose Program Access and Defaults. Open up whichever configuration is currently selected and change the default web browser selection to something other that what is currently chosen. Click OK, then test the link in Outlook.
This worked for me. YMMV.
(If you used this post to help fix this issue on your machine, let me know in the comments)