From Michael Knigge:
ForceDel is a lightweight and command line application that enables you to delete a file that is currently in use or locked by another application. It uses some exciting new APIs introduced with Microsoft Windows Vista to determine which processes have a specified file in use. With this information, ForceDel is able to close the file in the remote process address space and then delete the file. ForceDel features a simple syntax, which makes it very easy to use.