RunAs is a great way of temporarily elevating your privileges to admin level when you are logged on as an ordinary user. However, while you can right-click on .exe files and select Run As from the shortcut menu, this doesn't work with Windows Installer (.msi) packages.
Here's a workaround that makes this possible. Open Registry Editor as administrator and create the following key under HKCR\Msi.Package\shell:
Now set the default value of this key to the following:
Now create the following subkey:
Set the default value of this subkey to the following:
msiexec /i "%1"
Now you should be able to right-click on a .msi file and select Install As from the shortcut menu.