In Windows Media Player, go to Tools->Options and click the Media Library tab. There's an option called "Update my music files by retrieving missing information from the internet". It has several sub options, one of which is "Override existing information".
If this is on, and the thing is using otherwise default settings, then you run into a bit of an unusual situation. Windows Media Player attempts to be a sort of music categorization program. It will search your My Music directories and add files it finds to it's own library. If this update option is on, then it goes a step further and searches the internet in an attempt to identify your music and get tag information. It's not half bad at it either, but the point is that it does all this silently, in the background, whenever it is running.
And it's deceptively difficult to tell exactly when Windows Media Player is running. If you're watching a video in a browser window using Media Player, well, Media Player is running and doing all this. Tricky business, eh?
Now, iTunes is not immediately impacted by WMP retagging the music files. However, whenever you play a file, view it's artwork, do a Get Info on it, etc, iTunes re-reads the tag of the file and updates itself accordingly. Thus, it
appears like iTunes is changing the tag info, but what really happened is that WMP changed the tag info and iTunes only noticed the change when you did something to that track in iTunes.
Turn off the "Update" option in WMP and the problem goes away.
