Just a guess, and a hack too if it works.

Locked files can sometimes be renamed even though they are in use.

How about renaming the dll's if they exist, install the new dll's, install your software and register a small app to silently kill the renamed dll's at next reboot (but the user does not have to know).

And why not check what st**id a** application is locking the files? http://www.sysinternals.com/ntw2k/freeware/procexp.shtml

It's a wonderful world, so you might as well enjoy it.