How this tool works? TopWinPrio checks your windows once a second (that can be changed). The active window will receive a higher priority than normal. When you shift to another window, the previous window will return back to its default priority and the new active window will now get higher priority.
TopWinPrio is a free, lightweight application that can be downloaded at source (requires .Net framework 3.5 for Windows XP)
Note: TopWinPrio will not boost applications run with administrator rights. Do not use High priority on online games, it will conflict with your network driver’s priority and that will cause your game to lag.