If you get the error, "This operation has been cancelled due to restrictions in effect on this computer. Please contact your system administrator." when clicking on a hyperlink within an email, this means that there is no default internet browser (properly) registered in Windows.
To resolve this issue, you will need to assign a default browser via:
1. Windows Vista and Windows 7 - Start > Default Programs
2. Windows XP - Start > Control Panel > Set Program Access and Defaults
3. Windows 8 - Press the Windows key and Q > Type "default programs" and select it > Navigate to "Select my Default Programs" > Select your chosen browser > Select "Set This Program as Default"
When you have multiple browsers installed, it sometimes helps to set the other browser as the default, apply the changes and then set your preferred browser as the default again.
If you only have a single internet browser installed in Windows Vista or Windows 7, you can deselect all of the defaults for the browser by pressing "Choose defaults for this program", save the changes and then select them all again.