First of all, this is not a Powerbook specific question. It should be in a software forum. Second, I think you "dragged" it to the trash. I don't know how to "drug" anything anywhere.
As for the solution, do a get info on WMP and see if the application is locked. If you unlock it and still have the problem, just try rebooting your machine. You may still be running it or the system may still be using something in it. Rebooting while it is in the trash should fix this. If you still can't delete, check out Versiontracker for a tool that might help.
Steve