This error usually occurs when you have been editing a document directly from the Sharepoint and your system or the office application crash or lock up.
Whenever you edit a document from a Sharepoint site a temporary copy is saved in a hidden system folder, if this file is not cleaned out properly when the document is closed it can cause the document to always show as open for editing.
This is easily resolved by deleting all the files in the following folder (you may wish to back them up instead as it is possible for the versions there to be newer than the ones on the Sharepoint):
on XP C:\Documents and Settings\$USER$\Local Settings\Temporary Internet Files\Content.MSO
on Vista/7 C:\Users\$USER$\Local Settings\Temporary Internet Files\Content.MSO
In some cases, the above fix does not work. If that happenes the following steps can also be taken to remedy the situation:
Install the software update for web folders - http://www.microsoft.com/downloads/details.aspx?familyid=17C36612-632E-4C04-9382-987622ED1D64&displaylang=en
Disable IP6 on TCP/IP protocol
Change properties of affected Office app (Winword.exe or Excel.exe) to run in compatibility mode for Windows XP (SP2)
Create the registry key as detailed in this article - http://support.microsoft.com/kb/870853