Where can you download windows movie maker 5.1 transitions and affects?

There is no Windows Movie Maker v5.1. There is a Windows 5.1 build version, but I think the one you are looking to get transitions and effects for is the 2.1.4026 version (v2.6 technically speaking).

In any case, rehanfx.org has effect and transition downloads, but they are designed for Xp Movie Maker and are extremely hard to install on Windows Movie Maker (v2.6). Although not impossible from what I hear. So you might want to check that site out.

Otherwise, Blainesville has various transitions/effects/overlays you might be interested in that are much easier to download and install than Rehanfx. The URL for that site is:

http://movies.blainesville.com/2007/02/custom-speed-effects-for-movie-maker-xp.html