is it reasonable to automatically run an installer (for an activex control) as administrator on all machines in my domain, using an out-of-the-box windows (any version)?
It depends a lot on the context.
What do you mean by "run an installer as administrator on all machines"? How are you going to actually run it? By logging on to all machines and clicking on it? By sceduling a task? By using a script? By group policy?
Or are you asking exactly for this, i.e. "how can I run an installer as administrator on all machines in the domain?"?
Please give more details, it's just not possible to answer this question as it is presently stated.
Active X controls often need to be installed as administrator in order for Internet Explorer to download and install the Active X control to the special "Downloaded Program Files" folder (normally C:\WINNT\Downloaded Program Files - it is a super hidden folder in windows explorer). In addition several registry keys are written to ensure that the Active X control file version used locally is the same as the file version in the installation source URL. I believe IE checks this on every launch. If the file versions match no installation action is triggered.
In most cases I have been able to package Active X controls by capturing its installation while logged in and installing as an administrator and then creating an MSI file that can be run on the other machines and be installed by normal user accounts. See my answer here for some further details: http://forum.installsite.net/index.php?showtopic=20642&hl=