A few weeks ago, I started getting this annoying error when trying to click on links in Outlook 2010 messages. I have no idea how I caused it, but apparently it happens a lot.
There are a number of different things to check and try (like resetting your default browser, etc.).
What worked for me:
1.Start, click Run, type Regedit in the Open box, and then click OK.
2.Browse to HKEY_CURRENT_USER\Software\Classes\.html
3.Right click the value for the .html key and select Modify…
4.Change the value from whatever it is to “htmlfile”
Do this for the htm, xhtml, and shtml keys if they exist.
If the value is “htmlfile”, google for the other possible fixes.