Add the Trusted Installer account to the security tab of "%Windir%\Installer"
Try the steps here to restore your security settings back to setup defaults.
Note This solution does not apply to Windows XP Home Edition, or
Windows Vista Home Basic and Home Premium editions. To restore
security settings for Home editions, you can use the Fix it, System
Restore, or a backup. Click Start, click Run, type cmd, and then press
ENTER. In Windows XP, type the following command, and then press
ENTER: secedit /configure /cfg %windir%\repair\secsetup.inf /db
In Windows Vista, type the following command, and then press ENTER:
secedit /configure /cfg %windir%\inf\defltbase.inf /db defltbase.sdb
You receive a "Task is completed" message and a warning message that
something could not be done. You can safely ignore this message. For
more information about this message, see the
%windir%\Security\Logs\Scesrv.log file. Next steps After you run this
Microsoft Fix it (or complete these manual steps), standard user
accounts may no longer appear on the log on screen when you start your
computer or try to switch users. This occurs because standard user
accounts are removed from the Users group when you reset Windows
security settings. To add the affected users accounts back to the
Users group, follow these steps: Click Start, and then All Programs.
Or click Programs. Click Accessories, and then click Command Prompt
(Windows XP). Or right-click Command Prompt, and then click Run As
Administrator (Windows Vista). In the Command Prompt window, type net
users and then press ENTER. A list of user accounts is displayed. For
each accountname listed in the Command Prompt that is missing from the
log on or switch user screen, type the following command and then
net localgroup users accountname /add Now go to the "Did this fix the
problem?" section. Note In Windows Vista, the Defltbase.inf file is a
Security configuration template for the default security. You can view
the settings for this file in the following location:
For online Help about Secedit, click Start, click Run, type
%windir%\help\secedit.chm, and then press ENTER.
Definitely click the link and try the fixit tool first.