well if your logged in as admin... its not permission violations, it may be in use but that shouldn't stop it... possibly cause its a system file.
try hitting Ctrl+Shift+Escape and see whats running under your user name. look for said .dll or what not. anything that says 'you' under user name other than explorer.exe, devldr32.exe, and taskmgr.exe can be killed just don't kill any system or Local/Network services you may not want to kill IEXPLORER.exe cause its internet explorer.
and i doubt this will work unless you have permissions to the file but this will make it non system/hidden/readonly/ect
#include <window.h>
int main()
{
unsigned long fa = 0x00000080;
::SetFileAttributes("C:\\whateverdir\\admin.dll",f a);
// check to see if it worked
fa = ::GetFileAttributes("C:\\whateverdir\\admin.dll");
printf("%x\n",fa); // should print 0x00000080
return 0;
}