I am running Vista 64bit Ultimate, with MEW 11.03.
At one point, I was able to get MEW to start without having UAC interrupt me. At one point in the upgrade path from 11.00 to 11.03, this UAC business is back again.
I have a shortcut for MEW which has in the Properties Advanced tab, the Run as Administrator chosen.
Compatibility tab has for Privilege Level Run as Administrator chosen.
By default under Vista I believe anytime you have an application that isn’t signed, which Multi-Edit isn’t, and you try to run it as Administrator you will be prompted. So you can either uncheck the "Run as Administrator" option for the Multi-Edit shortcut or change the "UAC Behavior of the elevations prompt for administrators in Admin Approval Mode" to "Elevate without prompt" with the secpol.msc program.
Multi-Edit 2008 can run without admin privileges so turning the Run as … option off would probably be the best option. Earlier versions of Multi-Edit used a different compiler runtime library which caused the COM server to always be registered upon startup and thus forcing the need for Admin rights.