Without meaning to give half a solution - I don't know offhand what an efficient process would be, to suit your setup. It was just the thought that it's possible to wipe the software slate almost entirely clean, for testing purposes, by using another OS. One way to do that might be to install Ubuntu, e.g. on a second drive or partition, chuck Steam on it, and try comparing the performance of some applicable games which run on both platforms (Windows and Ubuntu). All the aforementioned Ubuntu stuff is done via the GUI, more or less like in Windows.