Double Click on My Computer (I'm assuming you're on XP), then click on Tools from the menu and select Folder Options. In the Folder Options window that opens, click on the File Types tab. Scroll down and see whether HTM and HTML file types are associated with Internet Explorer. If not, click on HTM, then click Change below. Scroll through the list of programs and find Internet Explorer. Then do the same for HTML.
You might also check your system for spyware. Do you have current Antivirus and Antispyware software installed?
Recently I downloaded Firefox and after working with it and some of the blocking features I have in place on IE decided to remove the program. From that point on I have not been able to click a link in Outlook and go to either IE or Firefox (reloaded after problem developed). I currently have fixed the problem to the point where I open IE and then open Outlook and am able to click a link. But would like to be able to go back to the original process.
I have set IE as the default browser, have attempted to go back to a previous day's settings and configuration.