try as grz said or get any advanced task menagers (whitch show what app is using whitch dll-s), or simples ctrl+alt+del, go to proceses tab, look for a name of that dll(maybe .exe has same name) end it, then try to delete
**edit
after googlin that dll, its mirc script, just a question, do u have any irc clinents, or did u visit argo chat recently, its created by mirc.....