This is caused by a Groupwise add-in. To fix this problem, remove the Groupwise add-in (either Gwxlus.xla for Groupwise 5.1 or Gwxl97.xla for Groupwise 5.2 and later).

Search for the Groupwise add-in in the following folders:

• C:\Program Files\Microsoft Office\Office\Xlstart
• C:\Program Files\Microsoft Office\Office10\Xlstart
• C:\Windows\Application Data\Microsoft\Excel\Xlstart
• C:\Documents and Settings\ User name \Application Data\Microsoft\Excel\XLSTART (whereUser name is your user name)
• C:\Winnt\Profiles\ User name \Application Data\Microsoft\Excel\Xlstart (where User name is your user name)
• Open the folder that is specified in Excel options, in the At startup, open all the files in box on the General tab of the Options dialog box.

http://support.microsoft.com/default.aspx?scid=kb;en-us;828484