replacer doesn't work

I've tried to get some custom themes and were told I needed to replace some dlls using a program called replacer.
It asks me to drag the uxtheme.dll into the window, then drag the new uxtheme.dll into the window after confirming with a Y and pressing enter I just get an error:

* uxtheme.dll:
- Replacing system file...
! Copy failed.

Press any key to quit.

I've turned off UAC and also tried to run it as administrator with no luck.
what's wrong? :S

I guess I've located what's causing it all, when I try to copy there I get an error which says something like "you need permission to perform this action" but I've even activated the hidden administrator and turned off UAC. How am I supposed to gain permission? :S

I'll try that one, really hope it will work now, getting desperate. tried everything now, also tried safe mode

Finally, thank you very much! Though it is annoying finding such a simple answer when it has annoyed me for so long

anyway to force windows into giving the hidden admin user all of trustedinstaller's rights so I don't have to do it manually whenever I wish to modify something? shouldn't compromise the security too much seeing I only use that admin for that purpose.