A lot of people had problems initially with the SP2 release because of one or more of the following reasons:
A: Their update file was corrupted (due to mass demand or bad connection)
B: They downloaded from a 3rd-party site instead of from the source (Microsoft)
C: They didn't read any documentation, notes, or anything, and messed themselves up.
D: They had illegal copies of XP to begin with, and SP2 blitzed them (as it should).
E: They had existing problems that SP2 revealed.
F: Their computers shouldn't have been on XP to begin with (borderline obsolete).
G: They had hardware that SP2 didn't support because the hardware manufacturers never bothered to keep tabs with Microsoft's partner release doco's.