Been trying to find my notes, to recall im sure i had a client with this issue. partially, had to upgrade then assist another on DEAD after he tried to upgrade.
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long story but
1. set computer to safe mode - annoying yes, but see if safe mode fixes the problem.....
2. if yes then continue
3. msconfig - hide all MS services
4. disable all - everything else....
5. reboot and see if this fixed you issue .... if yes
6. turn on 1 @ a time then reboot .... continue to turnon 1 @time. until the problem is back... takes a lot of time, but only sure way i know of find so you can fix.....