I've been having a browse round for links and I came across these two options that allow you to expand windows movie maker. The first link allows you to add effects and transitions but you will have to script them somehow. The second link costs a small amount of money but it's a lot easier than the first option, you just install and go. It's very, very easy and you get much much more. Anyway, check them both out.

1.http://msdn2.microsoft.com/en-us/library/ms867186.aspx
2.http://www.pixelan.com/mm/intro.htm