imageres.dll problem

Hello everyone..can somebody help me with my problem?? I was trying to modify the system file "imageres.dll"..The thing is, i want to replace it with a modified "imageres.dll" from desktop to its place but it was always saying that it can't be replaced because its open..So, wath i think would work is that, i renamed the ORIGINAL "imageres.dll" to "imageres2.dll" and then copied the modified...