So long as it's not in a set of files (like those in a VIDEO_TS folder) you should be able to safely rename the file.
If it's smallish, right click the file and then paste it into the same folder. The copy will be renamed automatically. Click the renamed file and see if it plays.
Edit -
Thats an idea traveler,but I would really rather have a folder called "Movies" with many different files in it.I want to know if I can alter a file name in such a way as not to hinder it.
But there's no reason you can't have a folder named Movies, and then sub-folders (for the actual content) that describe them.