Have a look at task manager, once the computer is booted and in a useable state.
Have a look at running processes - make sure you're looking at processes for all users.
I am a bit nuts over this, but I hate background processes running, hogging ram, that really aren't needed.
On my XP machines, with no actual programs running, I have 24 processes running, and xp is usually eating a couple of hundred Mb of ram.
I have seen people with 50, 60, 70 processes running when the machine is "idle" - and my god does a machine like that run slowly.
Have a look at a good services guide - you can disable 5 or so quite happily.
Then run msconfig, and disable any taskbar crap you don't need - I don't know what without having a look at the machine, but WinZip quick pick, quicktime, that kind of thing.
If you have inadvertently installed it, get rid of Google Desktop Search - it's horribly slow (unless you use it *all* the time, I know some people love it, but if you don't, just lose it).
Try and get the machine down to 30 processes or less, but especially indexing stuff like Google Desktop.
Your son hasn't accidentally put something like Seti@Home or Folding or something on, that is using the cpu all the time?
Personally I believe the install Windows every year is also a myth, but also like Piru, I tend not to install anything I don't know or need.
Hope that helps!
