When doing a a build (especially a batch-build) of many projects (in my case 40+), Visual Studio seems to have a tendency to grab handles to its own output files, and then not to release those handles. Pretty bad behaviour. And.. pretty annoying, too, if you constantly close and re-open your Visual Studio in order to force-close those handles. Oh, and one more thing – this bug seems to have been around since VS 2005, continued for VS 2008, and even haunts people with VS 2010. Yet again, congratulations MS on the many not-yet-fixed bugs in VS (googling on a bug of your own might show you that you are not alone, and that your specific problem has not been fixed by MS for a looong time! As long as it has been made known to MS).