Access Denied registering DLL under Windows 7 Embedded

In setting up a Windows 7 Embedded system, I need to run a couple old installers that register DLLs. These are failing--some with errors and some just exit without apparently doing anything. One installer reports that registering a DLL fails (even when logged in as Administrator). Trying regsvr32 to manually register the DLL reports that it received error 0x80070005 E_ACCESSDENIED.

More bizarrely, one of the installers is recent enough to throw up a UAC prompt when run as a normal user--and then proceeds to install without problems. The same installer fails while registering DLLs when run as administrator.

I suppose something is wrong with the administrator account, but I don't have enough Win7 experience to guess what (and without the explorer shell it's harder to poke around without knowing exactly what you're looking for). I don't need the administrator account for anything after this so I'd be happy just finding a way to force the older installer to run with UAC-elevated privileges.

Well, first try turning off UAC completely (and rebooting) to run the installer, which will correct the problem if the issue is caused by UAC.

The other possibility is that these old installers and/or .dlls are simply not (quite) compatible with Windows 7. In which case, I advise not trying to force it... because if you do manage to get the thing installed, you'll have to support a not-quite-compatible program on a not-quite-compatible OS until 6 months after you die.

Take if from someone's who been there, it's much better to just say nope, not compatible and make the bosses either buy something made to work on what you have, or stand up an old OS this app does work on, and use that to install it on. ("Buy something new" being a much better choice, of course.)