Windows Update
However - those of you who've altered your ntoskrnl file with utilities such as StyleXP will be having the same problem! About a week after M$ added an obligatory validation stage, I got lots of calls from people who I know have legit copies but couldn't get past this obstacle.
the answer - don't bother with the validation, go to Administrator Options on the Windows Update front page and from there go to the M$ Download site and do them manually - *some* will still require validation and you can't really do much about this if you have a modified ntoskrnl, unless you restore the original unmodified file and disable StyleXP or whatever is modifying that file. But most of the things you need like cumulative fixes, additions to the service pack and non-critical updates you can d/l manually no problem. And of course critical automatic updates are unaffected (mostly)
those of you with hookey copies, well if it won't validate you can hardly complain can you?