which program is using a certain file

[chrys_03]

How do i determine which program is using a certain file. here's an example, in my program files i have a folder named bonjour and there are two files in it

1.  mdnsNSP.dll
          Description - bonjour namespaceprovider
          Company - Apple Computer, inc

2.  mDNSResponder)
          Description - bonjour service
          Company - Apple computer, inc.

I'm not sure which program is using it so i tried to delete it so i'll know which program will not run when i reboot but the error message says access denied.

[cryhelp]

mDNSResponder: Apple Rendezvous client with MusicMagic Mixer uses to talk to TiVo. You can delete it without problems if you don't want to use the TiVo support
iTunes 5.0 loads Bonjour Service.