In short: Delete the file extend.dat.
Details: Please follow the following instructions:
To configure Windows to show all files
1. Do one of the following:
* In Windows 98/Me/2000, on the Windows desktop, double-click the My Computer icon.
* In Windows XP, click Start > My Computer.
2. Do one of the following:
* In Windows 98, click View > Folder Options.
* In Windows Me/2000/XP, click Tools > Folder Options.
3. On the View tab, uncheck Hide file extensions for known file types.
4. Do one of the following:
* In Windows 98, in the Advanced settings box, under the Hidden files folder, click Show all files.
* In Windows Me/2000/XP, under the Hidden files folder, uncheck Hide protected operating system files, and click Show hidden files and folders.
5. If you see a warning dialog box, click Yes.
6. Click Apply.
7. Click OK.
To repair the problem
1. Start Windows Explorer.
2. Locate and delete the Extend.dat file.
The default location of Extend.dat depends on your version of Windows:
* In Windows 98, the location is C:\Windows\Local Settings\Application Data\Microsoft\Outlook.
* In Windows NT 4.0, the location is %Userprofile%\Application Data\Microsoft\Outlook.
* In Windows 2000/XP, the location is C:\Documents And Settings\<your-login-name>\Local Settings\Application Data\Microsoft\Outlook.
Cheers
Vlk