Firstly run this - SFC /SCANNOW Command - System File Checker
That should replace any corrupt system files and put the original .dlls back.(you may need to run a couple of times depending on the errors,run until it comes back clean)
Than you will need to rebuild your icon cache - Icon Cache - Rebuild
What is the reason you cannot do this. At the end of the day you have little choice if you want the default icons back.
FYI-That theme/icon pack will have changed icons in more than just the imageres.dll which is likely why you still have custom icons showing