As long as you have a clean system free of any infections (use virus/spyware checkers, and cleanups) and u have the latest drivers for all devices in your PC, and software (some versions that are built before Service Pack 2 came out) are all updated,
after following those steps, you should have no problem installing Service Pack 2, and its an upgrade that leaves you feeling a bit safer while surfing the net since it covers many of the security holes.
If you are worried though about it damaging your system, do a backup of valuable files that you cannot ever lose (should be backed up anyway
) and then run it.
I think it saves the old version or something to get back to if something goes wrong (maybe in System Restore) thopugh if you press F8 at bootup to give you more options, one of them is to use old settings that worked, and this might also do it, so there are a few precautions you can take
I hope this helps