There is a program by MST Software that will tell you which process is using a file and preventing you access to it. The software is easy to use, its called MSTisUsedBy and once you run it all you have to do is drag the file in question into the gui window of mstisusedby and it will tell you which process or app is using it. Helpful for tracking down spyware/viruses and such. Here is a link

http://www.mstsoftware.com/c_mst_IsUsedBy.aspx