The worst problem I had was backups.....
I have norton "ghost", and if I backed-up, and changed
harddrive and then tried to restore it, with sp3, it always
gives an error about the swap file, and won't start windows.
It didn't do that with sp2. I haven't had to try it yet with sp4.
I also have 3 digitizing programs that just don't work with sp3,
that do with sp2. Again I haven't gotten to the point of checking
those yet with sp4. There are some other issuses with ie, and mp, and
sp3, but I don't fell like doing all that typing.