You know the drill - you've been asked to check why you cousin's computer is running slow. I was there yesterday. Being a Mac user since 2007 I haven't really dug deep in Windows internals in the past five years. Googling for answers reveals many, many different answers: broken registry, spyware, antivirus program, fragmented disk, turning of visual effects etc.

In this particular case I was asked to look at a two year old HP laptop with Vista. Windows was running incredibly slow and even opening up a new explorer window took almost a minute. I ended up doing everything of the above: running cc cleaner, defragmenting the disk, turning off visual effects, turning off norton and a bunch of other things people believe have an impact on Windows performance.

Now I'd like to understand this in depth. Is there a proper, "scientific" if you so will, way of debugging and understanding where the problem with a slow running Windows installation lies? (In my particular case this concerned Windows Vista but let's try to create general guide for XP and Windows 7 too).

To me, it seems wrong to just run a bunch of different tools without understanding the underlying cause of the error.