You don't say which version of Outlook you are using but I thought there were settings to enable or disable hyperlinks, possibly related to Junk Mail settings, I will have to check 2010. There is a setting for disabling hyperlinks is Phishing messages.
If it worked once, perhaps some update reset or added the setting and set it to not allow following links, if the setting is actually there.
I also remember something about a default browser being related to hyperlinks. If you check your default programs, do you have something set for default browser. Possibly there are settings in the Default Browser that could effect the links.
Are you getting any messages when you click the link?
Some of what I mention is in this link, so you...