UAP causing trouble to AutoConnect functionality built into I.E.

I am updating a custom RAS dialer to work on Windows Vista. It already works
fine on Windows XP, even as a limited user. The RAS phonebook is set to
execute a custom .dll, when the user opens up Internet Explorer. The
CustomRasDialDll is set to the custom .dll. This .dll launches the custom RAS
dialer to perform the necessary connection to the Internet. When a normal
user attempts this, CreateProcess fails, returning "Access is Denied".
Launching the Dialer does work when logged in as an Administrator, or if I
disable the User Account Protection feature.

What changes need to be made to the custom .dll to allow it to launch an
executable? I have read that adding a manifest can help executables, but I
haven't seen anything about .dlls. The process that loads the custom .dll
would be the Internet Explorer's executable, which I have no control over.