Našel jsem zmínku o tom, že tomu bránil Rootkit:
Reply 32: Security update for windows XP KB2633171 will not install
Togn replied 1 month ago
Ok. I solved my problems definitively. I had a rootkit virus (Rootkit.Win32.TDSS.tdl3), identified by the anti-rootkit utility TDSSKiller.exe (Kaspersky). This utility proposed automatically a cure. It found an infected volsnap.sys file, that used and locked permanently the ntoskrnl.exe file (SYSTEM). After the cure and a reboot, the update KB2633171 was installed without problems. I recommend ...

Zkus Windows proscanovat: http://support.kaspersky.com/faq/?qid=208283363