Task Sequence Command Line to Unregister DLL Not Working...Won't Run As Administrator

Question

Hi All,
I'm trying to unregister a dll by creating a Custom Task Sequence using the command line "%SystemRoot%\System32\regsvr32.exe" /s -u "%CommonProgramFiles%\Microsoft Shared\VGX\vgx.dll"
SCCM is reporting success, however the DLL isn't being unregistered on the client machines.  If I manually run the command on a client machine as administrator it works.  I have specified that same administrative user in the Custom Task Sequence. 
These are Windows 7 machines, so my hunch is it has something to do with UAC, but I'm kind of stumped what to do here but feel like I'm missing something obvious.
Thanks!

Answer 1

Thanks again Jason for sticking with me here.  We are running this on 64-bit Windows.  Okay, I fixed the command line so it now is: %windir%\sysnative\regsvr32.exe /s -u "%CommonProgramFiles%\Microsoft Shared\VGX\vgx.dll"
Once again, SCCM shows it successfully executed the task sequence, however the dll still didn't unregister.
Any other ideas what might be going on?  I really appreciate the help.
Here are the events for the advertisement from old to new in case they help:
Program started for advertisement "SEA2011B" ("SEA000E8" - "*").
Command line: "C:\Windows\SysWOW64\CCM\TSLauncher.exe"
Working directory: C:\Windows\system32\
User context: NT AUTHORITY\SYSTEM
The task sequence execution engine started execution of a task sequence.
The task sequence execution engine successfully completed the action (Run Command Line) in the group () with exit code 0
Action output: =======================[ smsswd.exe ] =======================
PackageID = ''
BaseVar   = '', ContinueOnError=''
SwdAction = '0001'
Command line for extension .exe is "%1" %*
Set command line: Run command line
Working dir 'not set'
Executing commandline: Run command line
Process completed with exit code 0
Command line returned 0.
The task sequence execution engine successfully completed a task sequence.