On my machine (Windows 7, 64 Bit) the TrustedInstaller.EXE is permanently active. The amount of used CPU changes permanently. 2 seconds 0 % then up to 50% (I have a dual core) for a second then down to 0.
Rebooting the machine did not help. Also a complete Virus scan was negative.
I have monitored the process with the Process Explorer tool. I can see that the process always access the same Registry key when it gets active. The key is
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Component Based Servicing\PackageDetect\Microsoft-Windows-IIS-WebServer-AddOn-2-Package~31bf3856ad364e35~amd64~~0.0.0.