I have a few questions before I can really respond, we’ve had similar issues with both mapped drives via NetServer and mapped drives using SNA Server.
1) What type of file is this? Flat file, CSV etc..
2) What application are you trying to open it with in Windows?
3) Have you tried copy/pasting the file to your windows box to see if it will open better locally?
4) How large is the file?
5) Is the file name in standard “old” DOS format?
6) Is this file created by a process on the system, or is it uploaded to the iSeries?
7) Most importantly, have you ever been able to open it before? If so, when did it stop working, and have there been any CUMEs or individual PTFs etc.. applied since then?
That should give everyone enough information to start trying to figure this out with you.