trouble using "is_file()"

I've got a d/e screen that collects a path and a file name. My script then attempts to verify that the file exists before moving on to do things with it. I've displayed the combined fields and they are showing correctly - ie, "c:\temp\emax.pdf" as well as "c:/temp/emax.pdf" . But when I use that var in "is_file($fname)" I get a false returned. But I know it exists there. Never used this function before, so I'm probably doing something stupid. btw - running on an XP machine with php 5.2