Replacing old Power Point DLL

Currently I have the DLL for Aspose Power Point in the bin directory of my project. The DLL, along with other DLL’s, are in Source Safe, under Visual Studio 2003. If I attempt to delete the DLL for version 1.4.4.0, so I can replace it with 1.5.4.0, I get this error message:
"Error deleting file “Aspose.PowerPoint.dll”. Unable to delete bin/Aspose.PowerPoint.dll. The process cannot access the file because it is being exclusively used by another process.
So here is my question. I have asked a couple of our “gurus” here and they are confused. Do you have any ideas what could be causing the error message, when I attempt to delete in in Solution Explorer under Visual Studio 2003 (where the source code is on Microsoft Source Safe)? Is there any way to identify the process exclusively using it?

Dear Randy,

http://www.sysinternals.com/ntw2k/freeware/listdlls.shtml

Thanks Alex. A very useful tool. One of our contracter gurus here gave me this google link at

http://www.google.com/search?client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial_s&hl=en&q=The+process+cannot+access+the+file%2C+because+it+is+being+used+by+another+process&btnG=Google+Search
I looked at one reference at http://www.kbalertz.com/kb_883825.aspx
The website suggested to delete the reference link and then add the new DLL. When I followed that advice, it worked. So I will share this for other forum readers who may run into similar issues.