Can't delete a file in System32 folder

The audio in MSN Messenger has stopped working, although the video feature still works. I've read on a MSN Messenger forum that a lot of other people are experiencing the same problem and have asked for help in resolving the problem. The reason given for the problem is a bad rtcdll.dll file that is put in the system32 folder by an updated version of Messenger. The solution offered is to replace the bad file with the rtcdll.dll version 5.1.2600.28. When I attempt to do this, it won't let me delete or replace the bad file. I get a message that the file is in use by another program or that it's a protected file. I don't have a program running that I know of that could be using that .dll file, and I have uninstalled MSN Messenger. How can I delete this file so I can replace it with a working version?