So did you solve the problem?
I had the exact same problem a few months ago after I reformated my laptop (before that it worked fine). After I tried many differents things I did something that worked for me: I unnistalled the base game, used CCleaner to clean up Windows registry, rebooted my PC and installed again The Sims 2 (without Maxis' patch) and then The Sims 2 Celebration Stuff (which is the only Stuff pack I own), and
voilĂ , no more "The update process did not complete succesfully" problem.
After that I started installing the Expansion Packs (University, Nightlife, Oper for Business...) and without any problem at all. I don't know why it worked, but I really tried too many things (and spent too many hours x.X I'm a bit of a stubborn, I don't like it when I can't fix something xD -which by the way, is the reason I had to reformat my PC in the first place, haha) and that was my solution.
I hope this helps as it helped me.
--I'm sorry for my english.