I ran across this issue previously and now im seeing this issue at another client. I guess i forgot to write down the fix and now i am documenting the fix again: please note that the fix also works on Citrix XenApp 6.5 (windows 2008 R2 )
Edit the registry manually
If hyperlinks in your Outlook do not work any longer after you’ve uninstalled Chrome, Firefox or any other application (e.g. HTML web editors) that open HTML files by default, changing the HTM/HTML associations in the registry may help.
Important! Please be very careful when making changes in the system registry. If you are working in a corporate environment, it might be a good idea to ask your system administrator or IT person for assistance.
Anyway, before modifying the registry, be sure to create a system restore point and backup your registry completely, just to be on the safe side. The following step-by-step instructions by Microsoft may be very helpful indeed:
Now that you have taken the necessary precautions, you are ready to proceed to making the changes.
1. On Windows 7 and Windows 8, click the Start button, type Regedit and hit Enter to open the Registry Editor. On Windows XP, click Start > Run and then type Regedit in the Open box.
2. The Registry Editor will open, and you browse to HKEY_CURRENT_USER\Software\Classes\.html. Verify that the Default value of this key is htmlfile.
3. If the Default value is ChromeHTML or FireFoxHTML (depending on which browser you have installed), right click it and select Modify…
4. Change the Default value to htmlfile.
5. Repeat steps 3 and 4 for .htm and .shtml keys.
6. Restart your computer for the changes to take effect.
The alternative way to make the same registry changes is to click the Start button and type the below command directly in the search line on Win 7 or Win 8. If you have an earlier Windows version, click Start > Run and then enter the command in the Open box.
REG ADD HKEY_CURRENT_USER\Software\Classes\.htm /ve /d htmlfile /f
Then enter a similar command for .htm and .shtml keys.
After making the above registry changes, make sure that Internet Explorer is set as your default browser.
Reset Internet Explorer settings
If the problem with links in your Outlook persists, try to reset Internet Explorer settings.
1. Verify that your Outlook is closed.
2. Start Internet Explorer, click the Tools icon 
and choose Internet Options.
3. Switch to the Advanced tab and click the Reset button (if you use Internet Explorer 6 or lower, you’ll find this option on the Programs tab).
4. The Reset Internet Explorer Settings window will open and you select the Delete personal settingscheckbox, then click Reset.
5. Click the Close button when the resetting process is completed.
6. Be sure to set Internet Explorer and Outlook as default programs, as we discussed earlier in this article.
7. Close and then open Internet Explorer anew and after that check whether hyperlinks are working again in your Outlook emails, tasks and other items.
Import a registry key from another computer
If recently you have upgraded to a newer version of Internet Explorer, the following registry key may be corrupted or missing: HKEY_Local_Machine\Software\Classes\htmlfile\shell\open\command
You can fix this by importing it from another healthy computer to the affected machine.
Okay, now that I’ve given a word of caution and you heard it (hopefully : ), head over to another computer where Outlook links work just fine and do the following:
1. Export the registry key from the computer that doesn’t have any problems with links in Outlook.
- Open the Registry Editor. As you remember, you need to click the Start button, type regedit and then press Enter.
- Find the following registry key:
HKEY_LOCAL_MACHINE\Software\Classes\htmlfile\shell\open\command - Right click the command subkey and select Export from the context menu.
Alternatively, on Windows 7 or Windows 8 you can switch to the File menu, and click Export… there. In earlier operating systems, the Export option may reside on the Registry menu.
- Type a file name that is easy for you to remember, e.g. “Exported key” and save the registry branch to some folder.
- Close Registry Editor.
2. Import the registry key to the problem computer.
This step is probably the easiest one we’ve performed today. Simply copy the exported registry key to the desktop (or any folder) on the affected computer, and then double-click the .reg file.
3. Make sure the Default value of the HKEY_CLASSES_ROOT \.html key is htmlfile.
To check this, click the Start button again, type regedit to open the Registry Editor, and then navigate to the HKEY_CLASSES_ROOT \.html key.
If the Default value of this registry key is other than htmlfile, modify it in the same way as we discussed in Editing the registry manually.
Well, you have spent quite a lot of time troubleshooting this issue and hopefully now hyperlinks in your Outlook work again without a problem. If against all odds the issue persists and you still cannot open links in Outlook, restore your system as the last resort.