64 bit sp2 breaks most subsequent updates

I've been using the windows update tool to manually keep security and bug
patches current on my laptop but every time (tried four times over
several months) I install sp2 it breaks most subsequent updates. After
installing sp2 most of the subsequent updates fail with a nondescript
error code that suggests corruption in system files when I try to install
them.

I use the system restore to go back to the point before sp2 is installed
and everything is OK again.

It passes the KB947821 update readiness tool without reporting any
problems. Installation of sp2 itself never reports any errors. it's
only subsequent updates that fail.

hardware: Gateway P-78 64bit laptop
OS: vista home premium 64 bit version

supplemental software:
visual studio professional 2005 (with 64 bit updates)
directX sdk (august 2009)
ffdshow
msoffice 97 professional
.net framework 1.0 sp3 developer
.net framework 2.0 (32 and 64 bit versions)
.net framework 3.5 sp1
java 6 update 17
nvidia opengl sdk 10
nvidia Cg toolkit may 2008

updates and patches:
I'd post the list if there was a way to actually cut and paste
the list.

First question: Why does sp2 cause updates to break?

Second question: How to fix it without wiping the system disk and
restoring to delivered condition? (use restore disks and start over seems
to be a too common suggestion in problems of windows corruption)


Suggestions?