Okay, thanks, got the problem solved. Terminated all running copies of Explorer and Internet Explorer using KillProcess before installing the DLLs. Then check to see if the DLL is still locked, then whine at the user if it's still locked.

I know this is the backwards way to go about it, and there are drawbacks to this method, but for our particular application (locked-down systems of fixed hardware/software) it actually is the best solution.