Overwright Dll in system folder

I have a dll which gets registered properly on installation, But when i made a few changes to dll, replaced in setup and tried to install old dll is not getting replaced by new one in system folder.
It is not any system dll also it is used by another application so i cannot delete it on uninstall.
I registered it using follwing code:

!insertmacro InstallLib REGDLL $ALREADY_INSTALLED  REBOOT_PROTECTED "C:\Folder1\Usercontrol1.ocx" "$SysDir\Usercontrol1.ocx" "$SysDir"

what can i do so that new dll will replace the old one in system folder.
Thanks

i never got this part really answered http://forums.winamp.com/showthread.php?t=216212
so i still use my own script

;----------------------------------------
;Usage:
; push dirname+dllname
; call unload
;rename existant dllname
;and delete on next reboot
Function unload
  Exch $R1
  Push $R2
  Push $R3
  ;$R1=oldname
  ;$R2=newname
  ;$R3=counter
  StrCpy $R3 0
  ;try unregister dll
  ExecWait '"$SYSDIR\regsvr32.exe" /u /s "$R1"'
  Sleep 300
  UnRegDLL $R1
  Sleep 300
  Delete $R1
  ;if file does not exist skip all
  IfFileExists $R1 0 done
  
  loop:
    StrCpy $R2 $R1.$R3.tmp
    IfFileExists $R2 loop1 found
      Delete $R2
      IfFileExists $R2 0 found
    loop1:
    IntOp $R3 $R3 + 1
    Goto loop
  found:
  ;dllname was found > now rename it
  Rename $R1 $R2
  ;delete old dll-file on next boot
  Delete /REBOOTOK $R2
  done:
  Pop $R3
  Pop $R2
  Pop $R1
FunctionEnd
;----------------------------------------