Spent hours trying to figure out why I couldnt check-out and check-in excel files or any files for that matter in SharePoint 2010. From a Windows 7, Office 2010, IE9 computer I never ran into issues, however with a Windows XP, Office 2007 and IE 8 I couldn’t. Kept getting the error “the document could not be checked out”. Ran fiddler traces and looked at ULS logs and really nothing pointed to any issues. Finally after many searches I found this article: TechNet http://social.technet.microsoft.com/Forums/en-US/sharepoint2010setup/thread/4e41fd23-1b8c-45e3-a4e8-fdba4e852317/ . Please see post from “Carlos_Shepardos” Sure enough I was missing a root site collection within my web app. So I didn’t have to install anything or change any settings on my client computers, just created a root site collection that the end user doesn’t even need access to. Hopefully if you catch yourself using Windows XP, Office 2007 and IE8, you will find my blog. I hope it works for you. 
