I had this problem last year trying to do the exact same thing. If I slipstreamed with any other program except Nlite it worked fine. At the time Nlite's creator said Sysprep was mainly used by companies who don't pay for his product so he wasn't going to fix it.(I don't blame him honestly) If I remember correctly it was determined that it was caused by Nlite repackaging those files. One thing I haven't tried that you might. Under Hotfixe, Addons and Update Packs (Maybe under Service Packs too). Click Advanced. Uncheck the first option to disable direct integration. Perhaps then it will leave the packs alone. Sorry I don't have a solution, I simply gave up on it.
