Remove reference of a DLL - IIS

Good night.

I am having an annoying problem with a DLL.

Well, i needed to make a test with two different versions of a DLL on an application on IIS.

I renamed the original dll Primavera.ERPOnline.Bso.Proxy.dll to _Primavera.ERPOnline.Bso.Proxy.dll, and copy the new version to the folder.

After all the tests i removed the new dll and renamed the _Primavera.ERPOnline.Bso.Proxy.dll to the original name.

I made an iisreset, and try to initialize the application but for some reason i get an error, and i cannot start correctly the application:

Exception message: Could not load file or assembly '_Primavera.ERPOnline.Bso.Proxy' or one of its dependencies. The process can not access the file because it is being used by another process. (Exception from HRESULT: 0x80070020

What do i need to do to "delete" that referece to that dll?

Thanks.

Please check with this http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx