I will be useful for people using xp or later version of OS.
One day I stuck with the problem when i was trying to delete DLL from my Folder. But it came with error which saying that DLL is used by other process. I looked on Task manage but could not find much.
Use Tasklist.exe command with option /m which list out all dlls
used by the each processes in memory.
Check the latest Process explorer from Microsoft.