Readability 5,
Since nobody more knowledgeable than me has offered a resolution yet, I'll have a go.
There is a possibility of finding what application is using the .dll with
Unlocker It's not always successful, I believe other programs like Process Explorer or A2 HijackFree may be better analysis tools, but I honestly don't know enough about them to recommend their use. (Can use myself, can't safely talk someone else through it.)
If you can right click on the .dll prior to attempting the SP3 install, it might tell you the "locking handle". First step then is to try and shut down that program, if it's not intrinsic to windows or update. (Hopefully it will be a third party software, that can easily be set to not start with Windows.)Then see if it's still locked. If not, presto.
If no luck there, I'd probably look at asking on the appropriate MS newsgroup/forum.