This is not an Outlook error. The most common cause is a corrupt registry key in Internet Explorer. The registry key found at
HKEY_Local_Machine\Software\Classes\htmlfile\shell\open\command
is either damaged or corrupt.
To fix it, try resetting Web settings in Internet Explorer. If this does not resolve the issue, set IE as the default browser following the steps
To Reset Web settings in Windows XP:
1.Open Internet Explorer.
2.On the Tools menu, click Internet Options.
3.Click the Programs tab, and then click the Reset button.
4.Under Internet programs, verify that the correct e-mail program is selected.
5.Click to select the Internet Explorer should check to see whether it is the default browser check box.
6.Click Apply, and then click OK.
If you receive a message when Internet Explorer starts telling you that IE is not currently your default browser, click Yes to make it your default.
If you are using Vista:
1.Open Internet Explorer.
2.On the Tools menu, click Internet Options.
3.Click the Advanced tab, and then click the Reset button.
4.On the Programs tab, under Internet programs, click Set Programs, then "Set your Default Programs", select your e-mail program and click "Set this program as Default". Repeat for Internet Explorer.
5.Click OK and close the dialogs.