Trouble with fopen() || Win2k Machine

Hey all. Quick question here that's driving me up the wall: Using fopen(), I'm easily able to open files for reading; however, when I attempt to open files for writing using fopen($file_path, "W+"), an error occurs. I'm able to use fopen($file_path, "a+"), but I need to be able to overwrite files; not write to the end of the file. Again, I'm testing on windows here.......... Any suggestions greatly appreciated, --Noah