Just spent 30 mins trying various alternatives to solve a problem on a relatives computer and want to bump up the 'solution' that worked.
Basically outlook 2007 under Vista Ultimate wasnt allowing you to click on urls or mailto links. Lots of the various 'incorrect solutions' ask you to reset defaults and check ie7 is the default browser. This didn't solve the problem.
Even the regedit for "current_computer" didn't solve the problem. It had to be solved with the .html registry setting as per the slipstick instructions above.
BTW turns out the culprit is an incorrect removal of Chrome browser.
- 1 for microsoft that doesn't give better "reports" on what the problem is..... asking home users to contact the system administrator is hardly a solution.