I found a file with
.DS_Store extension in one of my drives. How did it get there?
My Operating System is Windows 7 64-bit and to the best of my knowledge I never accessed an Apple computer nor was accessed by one.
That file is a proprietary Mac/OSX system file that holds attributes/meta-data about the folder it resides in.
These often appear in file shares that are accessed over the network by a Mac user; but that's not the only way you can get them. You may have gotten it in an archive (ie: a Zip file perhaps) you picked up someplace, which was originally created on a Mac.
I run into these all the time in archives we get from our graphic designers.
You're safe to just delete it.
More info: http://en.wikipedia.org/wiki/.DS_Store
For additional information on preventing them, perhaps check out these SU questions and answers (and others like them):