I searched my Vista machine for that .dll and did not find it either.
Are you pasting your password into the password field? If so, type it manually. Is the password complex - that is with characters other than letters and numbers and/or longer than 8 characters? If so, change it to a simple password temporarily at http://account.live.com and give activation a try.
I'm assuming that you confirmed the Subscription LiveID already..
In any event, one possible solution is to create a new User Account in Vista with Admin rights. Log into that UserID on your PC and activate OneCare. Once you've activated, you can log off, and log back in under you original ID and delete the new UserID if no further issues are encountered.
And, finally, you can contact support for help.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve