To preclude that from happening - investigate/invest in an Imaging
program (Drive Image, True Image, BootITng). Or at the least use
System Restore. Nothing that you ever install is guaranteed to be
issue free. The permutations of Hardware/Software & customizations
makes it impossible to have something work 100%, 100% of the
time.
I do understand your concern. But if a machine has proper drivers
& applications that are fully XP compliant - most Critical updates
shouldn't have a negative effect. Microsoft is well aware of the idea
that their patches "Break Stuff". That's essentially why SP2 is taking
such a long time to prove-in and release. The last thing they want is
some kind of slip up that lets a major issue get past them.
But when SP2 hits, this newsgroup will light up like a Pinball
machine with all the "I Can't -or- It' Doesn't work anymore" postings.
Mainly because people won't read the documentation on the changes.