Usually there is no problem as the service packs are accumulative, ie a
service pack always contains the previously released ones.

Regarding SP2 for XP, I have installed the RC1 release of SP2 over a WinXP
SP1 machine without any problems.

/Jonas

Its not a good idea. It can cause quite considerable problems in older
versions of windows (EG it would revert your IE version back some distance
and break functionality).

If you have a service pack installed, it is best to use the deinstallation
facility of the service pack.
Look in Add / Remove programs in the control panel.

The reason why it is not a good idea is that installing an older version
over a newer version does not remove the new files that came with the new
version and the associated "stuff".

If for some reason you have to go back then generally it is best to go
forward as soon as possible. You will lose all critical updates and other
fixes that have been installed.

If you are trying to address some other issue such as moving your system
disc from an old computer to a new one, or you have to replace the
motherboard with one that is not identical then you should do a Repair
Install.

- Tim

No you can't - like you can't squeeze toothpaste out of a tube and then push it back
(aka 1st Law of Thermodynamics ;)