20 Oct 2006 at 22:53 - 2
First of all, a little bit more info in a readable format would be nice. But here's my suggestion.
EDIT: I overlooked something in your post when you said that Gw.dat was already being used by another person or program. This solution should still work, but the problem may be something different than what I originally thought.
There's a problem with your Gw.dat file, that's for certain. What you should try is have the installer bypass that specific file by installing to a different location. For instance, instead of C:\Program Files\Guild Wars, try C:\Guild Wars. That may help you. And in my experience, installing from the download file on the website (Click on "Download the Client") seems to work better than installing from CD.
If the game works from there, then change any shortcuts and menu icons you're using to go to the new location instead of the old one.