Visual Studio SP1, MSBuild and Sitecore 5.3 are arguing

Having problems with Visual Studio 2005. While building my solution, Microsoft.Common.target, a MSBuild config-file tells me they want to delete several DLLs but isn’t allowed to do that. It seems to be a post built event. Can’t reproduce it directly on my Playground but I’ve the feeling that ist has somethign todo with the triangle ‘Web Application Project’, ‘Sitecore 5.3’ and MSBuild… Going to investigate it futher, stay tuned…