I still feel it is related to the hard drive. You could try turning off the windows paging file just to rule that out. Also, although you may not be aware of any background programs they may be starting when you start the computer. If you go into msconfig you could disable any non necessary programs from starting when you start windows just to have all the system resources available for the game. Otherwise perhaps the 5400 rpm drive may not be fast enough for the game to run fluidly.