Trouble installing nUI - Renaming Files

[Ratman1231]

Hi. I am hoping that someone can help me with what is probably a simple fix. When I try to update nUI, I follow the instructions and open my WoW folder, then attempt to rename my "WTF" and "Interface" files. I have had no trouble renaming the Interface file as "Interface.saved" but when I try to rename the WTF file, I keep getting an error message that my WTF file can not be renamed as it is in use. I have made sure that ever other file and program (at least as is feasible) is closed. Does anyone have any ideas or suggestions??
Thank you for your help. ~Bob